Migrate (and fix) GnomeDesktopThumbnail API here
I plan on moving and fixing the GnomeDesktopThumbnail API in gdk-pixbuf, as its demise was probably slightly exaggerated/optimistic.
Why not in GIO (with extension points) so that different types of thumbnail creation and storage can be used on different OSes? Because apart from the free desktop unices, the thumbnail representations used can't be consumed by anything but the native stack, and thus not GTK. For example, macos can generate and cache nsImage/cgImage/uiImage thumbnails, but neither gdk-pixbuf nor GTK can consume those.
macos image formats supported by the thumbnailers:
https://developer.apple.com/documentation/quicklookthumbnailing/qlthumbnailrepresentation
Windows thumbnail formats:
If that's agreeable, I'll write the API shortly.