it’s activitypub thing so lemmy as well. you sign up on one server, subscribe to community on different server, mods of the different server will decide they don’t like your server for any reason (e.g. explicitly because nods of your server didn’t blacklist some other server) amd you are out of that community. it sucks. imho something based on the old news system would be better (i.e. community exists independently and servers just subscribe to them or not).