Neither SMS, nor RCS, have built-in end-to-end encryption — and both should die a quick and timely death. iMessage to use end-to-end encryption, but it’s not open, and should also die. What messaging needs is an open modern standards, nothing more, nothing less.
RCS on Signal/Android does: https://www.gstatic.com/messages/papers/messages_e2ee.pdf
Am I missing something?
PS- Assuming that document is by Google, but unclear, what is the source that links to it?