Got a toast saying "downvotes disabled "

So you live in the cradle of Western civilization

No, any server instance could support sso as long as they agree on the protocol. If the one you're using now turned it on tomorrow you'd be able to use it elsewhere.

You could still make separate identities if you choose. At present you're still using a single one to interact across the fediverse (unless, of course, you make another one in the same way), so no additional loss of privacy

Tech question: I just signed into beehaw with a third party app for the first time. Downvote button is there. I wonder if it works? Is the disabling on the UI layer in the browser (like some subreddits did) or is it deeper?

Gonna try downvoting my own comment here to find out.