API
Disallowed saving a non-workshared cloud model to Plans folder in BIM 360 Docs via API.
Added two new static methods BasePoint.GetProjectBasePoint(Document doc) and BasePoint.GetSurveyPoint(Document doc) to get project base point and survey point of the document.
Added a static method InternalOrigin.Get(Document doc) to get the Internal Origin and read-only properties Position and SharedPosition to the InternalOrigin class.
Added LabelUtils.getLabelFor(BuiltInCategory, Language) to get the name of a category in any language supported by Revit.
Added an API class OffsetSurface to represent an offset surface.
Added API methods Mesh.GetNormal() and Mesh.GetNormals() to get the normals assigned to the mesh. Added new API properties Mesh.DistributionOfNormals and Mesh.NumberOfNormals indicating the distribution and number of normal vectors along the tessellated mesh surface.
Added the public API property Element.VersionGuid to obtain element version Guid in order to see whether this element has changed compared to the previous saved version of the document.
Added API class CurveUV to represent the notion of a curve in the 2D parameter space of a surface in 3D space.
Added API method ExportUtils.getNurbsSurfaceDataForSurface() that returns the necessary information to define a NURBS surface for a given HermiteSuface or RuledSuface. API method ExportUtils.getNurbsSurfaceDataForFace() is deprecated in Revit 2021 and will be removed in the next release of Revit.
Added API class Transform1D to represent an affine transform of 1D Euclidean space.
Fixed an issue that sometimes caused views in the model to be missing from the list of views in the Project Browser.
Added new API property Curve.IsClosed to describe whether a curve is closed.
Added API method CurveUV.As3DCurveInXYPlane() that returns a 3D curve lying in the XY plane in XYZ coordinates, representing the 2D curve with its UV coordinates identified with XY coordinates.
Added API method Edge.GetCurveUV() that calculates a 2D curve that represents the edge in the uv-parameter plane of one of the edge's faces.
Added API class Transform2D to represent an affine transform of 2D Euclidean space.
Improved the API documentation for appearance asset schemas classes.
Added API method Surface.GetBoundingBoxUV() to get the UV bounding box of the surface. Added new API property BoundingBoxUV.IsSet indicating whether the BoundingBoxUV is set or not.
Added API class EdgeEndPoint that represents the start or the end point of an Edge and API method SolidUtils.FindAllEdgeEndPointsAtVertex().
Added API method Edge.IsFlippedOnFace() to determines if this Edge's topological direction on the Face is opposite to its parametric direction.
Added new API property GeometryObject.Id to represent an integer id of this GeometryObject.
Added API class MathComparisonUtils that contains a set of mathematical utilities to compare doubles, using the internal tolerance.
Changed the default Document.Export API to include steel elements on IFC export so that it matches the default UI options.
Removed an obsolete API: ExternalResourceBrowserData::GetSubFolders() method.
Removed an obsolete API: ImageView.Create(Document, String) method.
Addressed several issues surrounding extensible storage to ensure more stability with this feature.
Added the ability to set a work plane in a 3D Assembly view via the API.

Analysis Visualization
Fixed an issue that caused a view that displays an analysis result to zoom too far out when controlling the zoom level with the Zoom To Fit command.

Annotation
Renamed the “Rotate with component” property of Section Head families to “Rotate text with component” and when checked, it will rotate the text in the Section Head family to orient to the angle of the section.

Category Visibility
Added the Infrastructure discipline filter in category filter options. 26 model categories/ sub-categories and 20 annotation categories were added to help manage bridge-related elements.

Content
Added 37 families for tunnel modelling and 13 families for windows rung as generic model in DACH content library.
Added 3 sample projects Architecture/Structure/MEP for JPN content library and 200 MEP families for JPN content library.

Content Browser
Added information to the Load Family tooltip on how to download Revit sample content.

Coordination Model
Added functionality to have a Coordination Model button appear in the contextual tab of the Ribbon with a Coordination Model instance selected to quickly access the dialog to manage the reference.

Design Option
Changed the order of Design Option Sets and Design Options to alphabetical order in all UI controls.

Dimensions
Fixed some issues with dimensions on cut edges that are created in dependent views.

Dynamo for Revit
Provided 10 new Dynamo Revit nodes related to Element functions.
Integrated Dynamo Revit 2.5 into Revit 2021.

Electrical Circuits
Added the ability to create a flexible circuit naming scheme.
Added Frame Size as an editable parameter for spare circuits.

Electrical Panel Schedule
Enabled property display of Circuit Number and Load Name for spare/space in the Property palette.
Added information to error when circuit is forced to disconnect due to mismatched panel during upgrade.
Enabled the Phase Label value to display for spare circuits in a panel schedule.
Updated French and UK Electrical and System project template to support the new circuit naming workflow.
Improved the behavior to show the panel schedule view nodes under sheet nodes in Project Browser.
Improved the behavior to support switch phase for 1- and 2-pole circuits in switchboard schedule.
Added the ability to specify circuit quantity on a switchboard instead of using the max number of single pole breakers.
Improved the single phase L-N panelboard workflow with proper load and phasing propagation.

Energy Model
Added the Peak Latent Cooling Load parameter to analytical spaces, which is populated by running a systems analysis. Added additional parameters on analytical spaces for future use; Heating Set Point, Cooling Set Point, Humidification Set Point, and Dehumidification Set Point. These will display as Not Available until implemented in the future.
Improved the naming of analytical spaces by adding a unique number to the end.

Export DWG/DXF
Fixed an issue where exporting some patterns to DWG creates hatches that don't report the area in AutoCAD.

Family
Added the ability to allow a void to cut geometry by family type or instance parameters.

Fill Patterns
Improved stability when encountering an invalid fill pattern contained in a CAD import or link.
Improved file performance and usability by removing all duplicate fill patterns when a project or family is opened in the current version and preventing creation of duplicates going forward.

Graphics
Added a Show button to Manage Links to find an image or PDF in project.
Added the ability to sort the list when managing Raster Images and PDFs.
Added the ability to multi-select to reload or remove Raster Images and PDFs.
Improved display in Realistic views lit with LED lighting families.
Improved performance when navigating a view with large numbers of rpc content in the view.

Help
Updated the tooltips for Plan View instance parameters (Range: Base Level, Range: Top Level, and Underlay Orientation) to improve users' understanding of the underlay range for a plan view.

IFC
Allowed Revit roofs with the IfcExportAs shared parameter to be exported to any supported IFC entity.
Improved parsing of class names in the IFC export mapping table.
Fixed an issue where the R parameter of rebar would export to IFC as the minimum bending radius instead of the actual bending radius of the bar.

Import/Export Image
Added new property SourceType and new constructor in API ImageTypeOptions.
Added new method Unload to API ImageType.
Added the ability to manage Raster Image and PDF Links and Imports in the Manage Links dialog, replacing Manage Images.
Added the ability to Link and Unload a Raster Image or PDF in addition to the Import option.

Import/Export Other
Added support for importing Sketchup 2019 files.

Import/Link DWG/DXF/DGN
Significantly improved performance while importing some DWG files stored with Desktop connector.

Install
Added sample projects from the Japanese User Group
Removed the RevitSDK executable from the installation media.
Removed the creation of the Worksharing Monitor desktop shortcut.

Join Geometry
Improved the Join Geometry command so it allows joining two structural columns.

Link Topography
Updated the style of error messages in the Link Topography dialog to be consist with Revit Home.
Improved the navigation experience and added project search function when selecting a BIM 360 account and project in the Link Topography dialog.
Added Decimal US survey feet as a length unit.

MEP Analytical Systems
Added the latent cooling load on analytical space after systems analysis.
Corrected the parameter name from "Thermal mass" to "Thermal Mass."
Improved the accuracy of the analytical properties for Mass Glazing and Mass Skylight conceptual types.
Added actual construction data to conceptual construction types.

MEP Fabrication
Improved the user experience when using the Revit Extension for MEP Fabrication by moving the buttons to more discoverable locations.
Improved the Reload Configuration feature to maintain changes to services and one-off parts when reloading after making changes.

MEP Miscellaneous
Added support for P&ID Modeler to connect with the P&ID project that is shared in BIM 360 Docs.

Macros
Improved Revit Macro stability and reliability with various .NET Framework versions.

Materials
Improved performance when opening the Material dialog when textures are placed on a remote server.
Allowed API users to read color values from some imported SAT, Rhino, and Sketchup files.
Fixed an issue where material thumbnails are regenerated unnecessarily when re-opening the Material dialog.
Improved stability when using dimension and material related features.
Refactored the way Revit interacts with RPC content, IES file parsing and material swatch scenes for easy maintenance and avoiding security issues.

Mechanical Systems
Added new units for Piping Flow - liters per hour and cubic feet per hour.

Parameters
Integrated Forge parameters into the Revit code base.

Precast
Added ability to preserve manual modifications on precast assemblies when doing changes to  the precast assemblies.
Added support for precast related content for all Revit supported languages.
Provided ability to visualize the progress of the main precast commands and also ability to cancel them during execution.
Added ability to control the modelling modifications on precast assemblies.
Integrated the "Structural Precast Extension for Revit" features into Revit.

Project Browser
Fixed an issue causing text in the Sheet Title dialog to get cut off when localized to some languages.

RayTrace
Interactive Ray Trace view mode has been removed and is no longer available.

Reinforcement
Added the ability to override the hook length for shape driven and free form rebar.
Added the ability to set the End Treatment for rebar without the presence of a rebar coupler.
Added the ability to use rebar couplers for connecting concentric arc bars.
Improved the display of bar handles for varying rebar sets during constraints editing.
Added the ability to model 3D rebar shapes with rotated hooks and extract full fabrication data.
Enabled solid and unobscured representation for standard, custom and bent fabric sheets in 3D views.
Fixed an issue where a multi-planar bar would only export half of the bar geometry to IFC.
Added the ability to dimension to the end of the rebar hook segment.

Revit Cloud Model (workshared)
Removed access to Plans folder in BIM 360 projects when initiating a workshared or non-workshared Revit Cloud model. The other functionality, such as linking and opening models from the Plans folder, remains the same.
Added dialogs to handle when user exceeds Active Window or Max Checkout Element limits.
Enabled BIM 360 Docs account and project switcher in Save as Cloud Model dialog.
Enabled hosting cloud models on Europe data centers.

Revit Home
Improved the experience for Revit Home to provide searching of BIM 360 projects across different accounts, four new learning content links in the left pane, and a direct link to the BIM 360 project page.

Route Analysis
Fixed an issue with the way the architectural sample model was laid out to allow it to work better with Path of Travel calculations.
Improved performance calculating the From Room and To Room parameters for a Path of Travel element.
Added a Dynamo Player script, to calculate and draw a Path of Travel showing the longest path to the closest of two egress doors in a floor plan.
Enhanced the behavior of a Path of Travel analysis so that when the whole path is moved, it will update itself to respect the current obstacles around it.
Fixed an issue that caused the Path of Travel to fail when optimizing a route around a family containing complex geometry.

Schedules
Fixed an issue that the schedule cell highlight might be incorrect when the error dialog displays.
Added the ability to set the rows to display in customized contrasting colors to make it easier to follow them across the schedule when reading wide schedules on sheets.

Shared Coordinates
Added the ability to turn off the visibility of Project Base Point, Survey Point, and Internal Origin for linked Revit models during project upgrade when the display settings of Revit links was set to custom.
Added the ability to hide Project Base Point, Survey Point and Internal Origin from host model and linked models when they were outside crop region of current view.
Fixed an issue where incorrect insertion point was used by using “Manual - Base point“ positioning option when linking an RVT project.

Steel Connections
Added support for SQL 2014 Update 3 starting with Revit 2021.
Enabled the highlight of input member during the creation and edit sketch for steel elements.
A midpoint and a center line were added to steel plates so that they can be more precisely positioned and moved, and accurate dimensions can reference the center of plates.
Fixed the way custom connections that result from broken beam end to end standard connections are applied on cases where the order of creation of the beams is different from the beams where the custom connection was defined.
Enabled creation of steel plates, bolts, anchors, shear studs, holes and contour cuts in vertical views (sections and elevations).
Added steel stiffeners, consisting of one/two steel plates reinforcing a beam/column section, welded to the input beam/column.
Corrected the size of the viewport of a section detail view in a sheet when the detail does not include a connection box, but the detail cut planes are intersecting that connection box.
Improved the behavior of standalone steel elements such as bolts, anchors, shear studs, welds that are not part of steel connections so that, when copying them, this action is completed in the background.
Enabled editing of contour cut boundaries and shapes of bolt/anchor/shear studs/holes patterns created on steel beams and plates.
Provided an easy and safe way for 3rd party developers and partners to install their new steel connections in Revit through API. Provided new API methods to create generic connections and mark steel elements changes through API. Created new documents to make it easy for API developers to build their own connections in Revit.
Fixed an issue that prevented structural connections to control hidden lines by the structural connections category in Visibility Graphics.

Steel Connections for Dynamo
Added the nodes of the Autodesk Steel Connections 2020 package into Dynamo for Revit.

Steel Custom Connections
Improved the behavior of custom connections created on two non-intersecting input beams when changing the size, position, or orientation of the main input beam.
Fixed a number of issues related to steel framing elements section getting lost in particular circumstances.

Steel Fabrication Beam
Fixed an issue that corrected the style of steel fabrication shape when the beam has a sub-category (e.g. girder) and its symbolic representation in the Revit family was set to the From Family option.

Steel Fabrication Elements
Improved the warning message that shows up when cancelling a steel plate sketch edit where changes will be discarded so that the message mentions that the sketch belongs to a steel plate.
Enabled the circle shape in Draw panel for contour cuts created on steel elements.
Fixed an issue in which the Exact Weight parameter was not updated inside the schedule for steel elements when the density value was updated.

Steel Modifiers
Adjusted the Radius and Boring out parameters of the contour cut to become disabled when the contour cut shape doesn't have the same fillet radius in all corners.
More control over steel fabrication parts and feature. Being able to edit contour cuts, bolts, anchors, shear studs and holes and drill patterns

Units
Updated Revit's internal handling of measurement units and corrected conversion factors.

View
Fixed an issue causing the Crop Region Size dialog in perspective views to incorrectly change the height value when inputting a new height value and checking the Scale radio button.

View Filters
Added a new column called "Enable in View" to the Filters tab of the Visibility Graphics dialog, in order to quickly turn on/off a filter action in the view.

Wall
Slanted walls can now be created using the standard wall tool.

Worksharing
When performing reload latest/sync with central, in ambiguous cases priority is given to local changes rather than to elements with smaller ids (created earlier).
Fixed the root causes of issues that cause deadlocks and data loss when worksharing with MEP elements.