Because iCloud Keychain sync is how each person can have one key for others to verify, rather than a separate key for every device they are logged into iMessage

Because iCloud Keychain sync is how each person can have one key for others to verify, rather than a separate key for every device they are logged into iMessage with. It’s described in more detail here: https://security.apple.com/blog/imessage-contact-key-verification/ https://security.apple.com/blog/imessage-contact-key-verific... (This is end-to-end encrypted, by the way; Apple can’t get at people’s private keys.) And this is a new protocol, so no surprise it doesn’t work with older operating systems. (It doesn’t say you have to remove your Apple ID completely, just log out of iMessage.)