So if I understand this right, you pick a server and your server account can post on that server and any servers that that server federates with.
So what happens to your account if the server you joined goes down? Yes, you could always create a new account somewhere else, but you lose all your followed communities and post and comment histories as well.
Well, if it was ages ago, instances may have gotten wise to the fact that your account is gone and deleted the posts by now.
No, I can go to instances I know my previous home instance federated with and read my old posts and comments right now.