Planning for the day when we have to bulk-disable a pubkey algorithm for being broken: I wonder if sshd needs an way to track in its own state, not just in logs, which remote IPs have successfully locally authenticated as which users? So that we can have a mode for "trust this algorithm P1 only if we've already (recently?) seen a successful login for that target user from that IP"; also "if and only if there does not exist an SSH key of pubkey type P2 for this user".