Please be aware there will be a one-time conversion of cached thumbnails and waveform data when you run this. The old database will be kept so if you downgrade or run an older version you still have a cache to rely on. You can remove either db.sqlite3 (old) or thumbnails (new) in the app data directory to cleanup (old) or re-run the conversion (new).

New

Added support for WebP export with presets for still sequence and animation.
Added Properties > Measure Video Quality... using VMAF. To use this, you first open the reference video, choose this new menu item in Properties, and then choose the video you want to measure with the file dialog that opens. If you want to use the current project for the reference, you should export it lossless and use that for reference.
Added a new video filter GPS Text (contributed by Daniel F). This renders data from a GPX file as text.
Added a new video filter Reflect.
Added a new Deband video filter (contributed by Austin B).
Added a warning dialog before File > Export Frame... from a proxy file.
Added support for keyframes to Mask: Simple Shape > Rotation.
Added support for the operating system clipboard when copying and pasting filters.
Added support for the operating system clipboard when cut, copy, paste, overwrite or append a clip.
Added a dialog when Timeline > Track Operations > Insert Track on the top audio track to choose audio or video.
Added handles to the sides of the rectangle on-screen control (VUI) (contributed by TRA).
Note: These only appears when the rectangle can be freely resized without aspect ratio lock.

Changes

Changed the storage for thumbnails and waveforms to use files instead of database. This improves performance when running multiple instances of Shotcut at the same time.
Changed Time Remap to limit input values to the maximum duration of the clip.
Changed to set the file date on proxy files to match original file.
Improved reliability of Time Remap status/feedback info.
Improved the speed setting in the Time Remap filter with an option to either modify or lock the input time.

Fixes

Fixed possible crash when using Audio Waveform Visualization filter.
Fixed Mask: From File > Custom... might lose track of its file.
Fixed Text: Simple > File date broken in v21.05.
Fixed Timer filter's new Speed parameter not scaling Duration.
Fixed a possible crash reading a YUV 4:2:2 uncompressed video.
Fixed some clips' duration in project XML saved in frame units instead of seconds.
Improved player grid display and snapping when zoomed.
Fixed Timer start and end buttons when filter in point is set.
Fixed quality/rate control with libx264rgb in Export > Codec.
Fixed undo simple keyframes in Size, Position & Rotate turned on advanced keyframes.
Fixed undo followed by redo when trimming clips on Timeline may not be correct.
Fixed a crash moving multiple clips to the beginning with Ripple All Tracks on.
Fixed changing Time Remap > Enable pitch compensation in disables Zoom keyframe Values.
Fixed file paths with special characters not working in filters such as Mask: From File.
Fixed exported HEVC MP4 and MOV files are not playable with macOS QuickTime Player or iOS.
Fixed Properties > Reset for an image.
Fixed a possible crash when a transition is removed during trimming on the Timeline.
Fixed keyframes may be dragged past another.