[dwaite]

I think iMessage is still using older attestations, but generally an attestation of this sort (App Attest, Play Protect API) represent a chain of the hardware, boot process, OS and application.

So iMessage is not going to be willing to hand out private keys or negotiate them for a third party application, and Beeper will not be trusted to register a private key itself.

Android iMessage support would be weird because there is no iMessage application - there is an application which lets you send SMS and to upgrade to MMS or iMessage when available. So, if there ever was an official Messages app for Android, I would somewhat expect it to also offer to take over being the default application for SMS/MMS.