Oh right, btw !GNUsocial. Noone using old versions can now unsubscribe to anyone using new versions (so the remote party understands it). A discussion on IRC made us realise you could do a replay attack on certain salmon slaps, including the "unfollow" verb. So I fixed the bug.

(the issue was that there was no designated target for the unfollow in the slap that was sent, so you could just direct/copy to to another salmon endpoint and successfully repeat the unfollow against some other identity.)