If you need .avif test image files you can download some from this page - AVIF browser test page: AVIF support in Chrome, Firefox, Edge… – libre-software.net – Linux, Firefox, LibreOffice
Gwenview or Krita do not show avif files for me either, Firefox from the repositories renders them okay, but it is a web browser and not an image tool.
In KDE’s kimageformats CMakeFile it lists libavif as an optional dependency with a target option in the imageformats CMakeFile , i.e. if it is found it will add avif format support and if not present support will not be added.
However, it is listed in the OMLx kf6-kimageformats spec file as a required build dependency making one think it should be being compiled with the support baked in but something else may be going on there.