@matcha_addict There are very few drawbacks (assuming it's implemented in a way that doesn't break things). That's why it's part of two of the big three social protocols (Nostr & AT/BlueSky) and Activity Pub might get it soon.
I've written about and participated in discussions about implementing identities not controlled at the instance level and discussed bridges that connect activity pub to other protocols. The one major drawback people tend to bring up is moderation, but moderation is not effected like some people think it could be. Just like a PGP key doesn't force Gmail to host a user's email and a domain doesn't force Dreamhost to host a blog, even if identities are separated from instances an individual instance can still ban a user from participating in that instance or prevent other instances from interacting with your instance. The only difference is that if an instance goes down or bans a user the user can pick up and move to a different instance instead of having their account nuked. As somebody who lost a profile due to a SQL database breaking it would have been really nice to have been able to continue.
Also, in the thread here I heard a few people talking about it negating communities. We already can communicate with remote servers, I'm not fully sure where the argument that independent-from-instance-identities will break communities comes from. If something like nomadic identities are implemented, which again, they may be, your account will still be largely focused on one instance.
Say you're an arborist and join an arborist Mastodon community. You're still a part of the community, and your account is centralized there until you say otherwise. Yes, you can reply to a lemmy post or peertube post by authenticating on one of those instances, but you can already do that (there's just a lot of jank since Activity Pub's monolithic servers often have a hard time understanding each other). Yes, say you reply to a lemmy post about beekeeping that would show up in the local insatance timeline (assuming remotely authenticated posts are allowed to show up in the timeline), but again not only can you already do that, but it's not like you'd expect an aborist focused instance would have ONLY aborist focused discussions.
Lol, I hope I was coherent. I just misinterpreted a bottle of bottle of lime infused liquor as 30 proof instead of 30% ethanol so I consumed a little more than I expected. Anyway, regardless, personally consider identities separated from servers/instances a very big pro, with very little drawbacks (if implemented in a way that does not break existing implementations).
@SorteKanin I know above he mentioned creating an account and then using it on anther platform like creating one on lemmy and then using it with something like Mastodon or Writefreely. If that was all he was asking about then Nomadic Identities would make that possible, though yeah if I just misinterpreted what he asked and we're talking completely disassociated (private key only instead of Zot's private key underneath a domain based username) then yeah nomadic identities wouldn't be quite what he was looking for.