Isn’t there a way to defederate but leave history intact? The fact that no new posts are coming from that source doesn’t mean there aren’t useful posts from when they were active/federated (that some users may have saved, etc.). As a general rule, the loss of information seems to me like a bad thing. Defederating should prevent new posts from reaching other servers, but not retroactively change what was already there.
As for new users subscribing: is should be something baked in the subscription mechanism, not letting you subscribe to something from a (currently) defederated server.