in the activitypub protocol, user accounts are registered on servers, so servers can delete accounts, and server-to-server migration of accounts requires the consent of both servers. so activitypub has no "nomadic identity" like nostr, or secure scuttlebutt.

see also

• https://socialhub.activitypub.rocks/t/nomadic-identity-for-the-fediverse/2101
• https://github.com/nostr-protocol/nostr#the-problem-with-mastodon-and-similar-programs