I wonder if there's utility in having some sort of "degrees of association" ranking system for dealing with spam accounts. Whereas like the 'Web of Trust' model originally envisioned for PGP, but in this case, not being about asserting legitimacy of real identity, instead just a "not a bot" rating.
Mainly where you'd endorse someone else's account as 'not a bot' on some sort of scale of: recent online acquaintance, long-time online friend, or have met in-person plenty. The risk of course is that such a thing could be abused for datamining, although, some of that could already be heuristically inferred (follower status + frequency of interaction/replies).
This is just a musing of distantly watching the happenings of Nostr in dealing with spam, and seeing if there's ideas that could be implemented in ActivityPub-land first before implementations of key-based portable identities are more widespread.