Can a admin delete every community and its posts from an instance on its end? For example, if a instance that was federated to your instance goes offline forever and is not going back online, can the admin delete all communities and posts from that instance that is federated to your instance?
I don’t think so, but as others have said it’s also easily doable directly via the database. It’s pretty rare that one needs to purge another instance. I’ve mostly only purged spam users.