AFAIK, we have .nomedia, .trackerignore and .backup-metadata (https://forum.sailfishos.org/t/newly-created-images-displayed-twice-in-the-gallery/9357)
They seem to be synonyms to exclude a dir from tracker indexing.
The simplest to implement, as starting point, would perhaps be having files like .nopicture, .nomusic, .novideo....
We could place those files to selectively exclude directories from indexig.