U2F and OpenSSH
As of OpenSSH 8.2, OpenSSH now supports U2F for authentication.
This is a totally separate authentication mechanism entirely from say, ED25519 host keys with a second factor; here, the U2F (e.g. Yubikey) is / has the key on it.
To use it though, both sides (server and client) have to be recent enough to support this form of login.