It wouldn't matter much about the clients, since the servers decide what it'll accept through federation.
On the topic of URIs as identifiers instead of cryptographic signatures: that's where some of the ideas brought up in 'BlueSky' project was aiming at, and was trying to invent an entirely new protocol versus implementing some concepts atop ActivityPub ( https://atproto.com/docs ).
For example, there's nothing in the ActivityPub spec that says anything about the @user@host ID concept, that's just something tacked on from WebFinger. I'm sure someone could devise supplementary extensions to make that work, and some of that has been part of recent discussions in the recent fediverse hackathon (http://hackiverse.org/)