I am talking about when instance X defederates with instance Y, so the users on X cannot see content from Y, even if they took no part in the decision.
Well, obviously you’re not going to be on a platform you dislike. I have signed up for Mastodon several times, but always ended up finding the instance dependency off-putting. I wish for a protocol where the instance mostly doesn’t matter and you can trivially switch to a different one, like with e-mail.
E-mail providers allow you to use a custom domain, so if the one you’re using suddenly goes away, you can just point your DNS records at the new one and that’s it. If the ActivityPub instance you’re using suddenly goes away, you can say goodbye to your account.