RTS is the SMS replacement and is quite old. And as evidence it works, I direct your attention to the ubiquitous iMessage, which only "couldn't" interop with Android because Apple was trying the vendor lock-in approach and actively enshittifying its own users' experience and blaming it on Android users.
Assuming you mean RCS, to which Google built a proprietary E2EE extension, remains the only client with no API to build off of and if I recall requires play attestation.
Hardly a complete solution, it devolved into a Google power grab, in my opinion.
For once I half agree with apple when they say its less secure, as apple-to-google comms are not E2EE like they are for google-to-google. While that is fixable with a hack its still a hack, not part of the protocol.
