Okay, this has given me a new idea.

These should be contact messages not about and should be handled inside of ssb-friends.

To merge feeds from your devices, you make them follow the other devices with sameAs: true in the contact message.

If two or more feeds have matching sameAs contact messages then merge the profiles together.

Then what about subjective identities?

I would like to also support a case where a feed follows another feed with sameAs but the other feed does not follow them back. For example in the lost key case, @Dominic would follow @dead_Dominic with sameAs: true.

If someone looked at @dead_Dominic, they would not see anything different. However if they looked at @Dominic's new profile, they would see an assertion saying that this other feed was also him.

This means that someone can't affect another person's profile display, but they can say things about other profiles from their one. People can already do this, no real change.

But the big benefit for @Dominic would be that he now receives mentions for his old identity in his client.

Other people could agree with his assertion, and publish a {type: 'contact', contact: '@dead_Dominic_KEY', sameAs: '@dominic_KEY'}). This would not appear on @dead_Dominic's profile in any way to anyone else. But now the person would see these accounts as merged. Potentially you could add a "x of your friends" agree that @Dominic is right about this assertion on his profile.

I'm less sold on this second part of the idea, but I am pretty confident that these messages should be contact instead of about.