From what I can tell, all the spam is originating from Mastodon servers running ancient versions with only one original user that stopped posting a few years ago. There are a lot of Mastodon servers just hanging out there, effectively being free replacements for temporary servers set up for spamming.

I’ve started defederating from spamming servers because banning accounts doesn’t help. So far it seems to work, but no doubt there will be another wave of spam at some point.