I’ve gotten the impression that if I down-vote a folksonomy tag that someone else has added, it could disappear (if it only had one vote previously). For obvious reasons I haven’t tested this much.
I couldn’t say if that’s what’s happening here.
It seems like the tags are gone completely and not just voted down. I’m guessing that this would happen if 1) the tags only had a single “(up)vote” for them and 2) the editor who added the tag removed the tag (or deleted themselves/got deleted, in which case I believe all tags, ratings, etc. associated with said editor also gets removed).
Ratings and tags are tied to user/editor accounts since it’s inevitably subjective (and thus personal) data. If the subject/person doesn’t want to be associated with MB anymore, any subjective data of theirs should also be removed (see e.g., the GDPR etc.).
You can look in old database dumps. I don’t remember exactly where they are (and I’m just about to board a plane, so can’t look myself), but try looking around our FTP server?