Is there any FOSS alternative messenger that handles SMS and RCS? Supposedly RCS is an open standard, but I am not aware of any FOSS messenger that has it. I’d like to move away from google messenger, but there doesn’t look like there is an option just yet.
No such thing.
Google has not, and probably never will, make it accessible to outside developers.
Since they decided to play host for rcs, they essentially own it now.
Yup. It’s an open standard, but that doesn’t mean they will let your client connect.
Even it wasn’t Google, you can’t self-host RCS backend. So it would have been carrier’s servers or someone else’s. Given that end-to-end encryption is not a part of the standard yet (and it won’t be mandated anyway), having an open source client would offer no privacy benefits.