Timothy Jacobs offers a “snapshot” of some of the changes to the REST API in WordPress 5.8. This includes the creation of several REST API endpoints dedicated to widget management.
Milana Cap also gathers some of the smaller changes that developers should know about. Some of these changes involve the removal of support for IE11, the _prime_post_caches() function, the delete_theme and deleted_theme hooks, and items related to WebP and the image_editor_output_format filter.
