KDE applications use kimageformats which use libavif to save AVIF image.
Unfortunately, libavif in debian was built without encoders, so it is able to read AVIF but not to write AVIF.
In order solve this problem, is is necessary to add new libaom (version 3 or 2) and/or rav1e to Debian. libavif doesn't work with very old versions of libaom.
When one of those encoders are added to Debian and libavif package rebuilt, you should be able to save AVIF images from KDE applications like spectacle, gwenview, etc.