ActivityPub can support portable identities, using the infrastructure and protocol that we already have: https://shadowfacts.net/2023/activitypub-portable-identity/

The crux of the issue with current software like Mastodon is that people and posts are identified by where they’re hosted, not who they belong to. It’s as if the URL for my blog post was https://5.161.136.163/2023/activitypub-portable-identity/. It moves the post from being controlled by me, the owner of shadowfacts.net, to my hosting provider who controls that IP address. But the web doesn’t work like that, and nor does ActivityPub have to.