Hrm, I’d have said that Mastodon, if anything, was leftier than bsky (unless you count Trump’s mastodon instance, but as it doesn’t federate you probably shouldn’t). It would certainly have a larger hard-left representation. Nostr, being vaguely crypto-flavoured, is messier.

Given that Twitter has been taken over by a far-right lunatic, one might reasonably expect the alternatives to lean a bit left.