We’ve closed registrations while this lemmy exploit thing is figured out. We will reopen after things get sorted out as far as how to mitigate this. New communities limited to admins only for the same reason and will be re-enabled when everything is sorted.
Defederation is significantly less common than with Mastodon and Lemmy and from my understanding is virtually impossible to isolate an instance. You can have servers/rooms running on multiple instances at any given time (and most do). I have never experienced an issue of true defederation where it was impossible to talk to someone else on another Matrix home server. I even was on a server one time which did defederate from Matrix.org. Even in that case I was still able to join matrix.org servers and talk to matrix.org users, though it was a touch more difficult (but barely).