I think that currently Keybase.io is the only thing trying to be universal, with their transparency log plus links to external profiles along with signed attestations for them.

But even that's still not quite what I'm looking for. There's no straightforward way to link arbitary protocol accounts / identities to it, outside of linking plain URL:s.

We need something a bit smarter than keybase that would actually allow you to maintain a single personal identifier across multiple protocols.