I know that some old albums of mine include <ARTISTSORT> tags
I'm trying to hunt them down
This is not a way to actually find populated <Sort Artist> tags, but it may help to mark those who are likely to be empty:
In the past it was also not possible to find tracks without <Album Artist> tags. But after some requests there was a change so that $IsNull(<Album Artist>,-,<Album Artist>) would work.
Perhaps it could be made possible to make that work for other tags like this one too.