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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-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.)

Probably requires a hardware-security-module for generating secure attestation keys.

Removing appleid from legacy mac and iphone because they don't have the hardware security module. So rather than having a broken imesssage experience on those devices, they decided its better for you to just unregister.

Beeper?

The contact key verification feature preexists Beeper’s iMessage support.

In fact, it seems like this would make Beeper and similar approaches an order of magnitude harder.

And I could totally see Apple making non-verified contacts' bubbles a different color sooner rather than later...