> How would federation solve this problem?

I'm not saying that federation solves the anonymity problem, I'm just saying that the current implementation includes Apple as a trust anchor for email address and phone number verification and issuance of corresponding certificates. My point is that in order to enable an open cross-platform solution, there would have to be some alternative mechanism to that.

What they could add is a sender-side option that makes sending completely anonymously. This would be possible without any change on the receiver side, but would require recipients to enable "allow all senders" mode.