Thanks @
Harald Eilertsen On Privacy versus Freedom | Matrix.orgA few years ago, back when Matrix was originally implementing end-to-end encryption, we asked Moxie (the project lead for Signal) whether he’d ever consider connecting Signal (then TextSecure) to Matrix. After all, one of Matrix’s goals is to be an interoperability layer between other communication silos, and one of the reasons for us using...
And also, linked from the above on "nomadic accounts":
MSC1228: Removing MXIDs from eventsBackground
We would like to be able to break the association between a user's ID (such as @richvdh:sw1v.org) and their activity in a room. The stretch goal is to also remove the association with server names, ...
Does seem like they're following the Zot footprint, with an asymmetric key as the gid.