Honestly, I wish this wasn't necessary. This is the worst thing about federation, in my opinion - inter-instance politics results in one instance admin defederating from another and all of the users of that instance, who may have no knowledge of nor care about the reason for the defederation, have all of their subscriptions disrupted. It'd be nice if it didn't remove already-subscribed communities, and instead just prevented them from showing up to other users under the Communities list / in the 'All' list.