Agreed. It’s one of the recurring requests (either for filter-out-older or filter-out-newer direction), such as:
which got inspired by
which points to some ways you could try to script this yourself (along with caveats on things like files repeatedly aging out then coming back into the backup – which might result in some excess uploads. Possibly you’ll get lucky and when the file goes “deleted-due-to-age”, its data blocks will linger awhile.