I'm still curious if this is even legal. It seems like a really good idea, but is Apple going to be able to sue over it? I almost feel like it could be covered under the reverse engineering clause, because it is meant to enable interoperability with another product. But Apple's terms of service already seem really hamstrung on what is and is not allowed. With the macOS SLA beginning with:

For use on Apple-branded Systems

Obviously iMessage isn't macOS, and I can't seem to find a specific terms of service for iMessage specifically, but it is running on it. Which is what would make this integration possible. So what makes me wonder if Apple's lawyers could find a clause there.

This sounds like The Onion, ridiculous.

This could easily be blocked by Apple

Are these a Matrix/Beeper bridge?

Goodbye, privacy

Whatever, Matrix/XMPP work with carrier-independent protocols.