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.