kurahaupo
commented
4 years ago Update 1: there's a good background explanation in this Reddit thread.
Update 2: a public RCS API was still non-existent as of March 2020.
(Side note: there's now a new MmsManager API)
This feature-request ticket consolidates numerous requests to add support for "Rich Communication Services" (aka "RCS", "Advanced Messaging", or "AM").
The heart of the issue is that there is no public API for accessing (sending, receiving, storing, or retrieving) RCS messages.
The Google API for retrieving SMS & MMS messages from a SIM does not handle RCS messaging at all, not even to give an indication that they exist, so it hasn't been feasible to implement RCS support in 3rd-party apps such as SMS Backup+.
Google added an entirely new API for RCS, but the new API was still "OEM only" as of July 2019 with the Android Q release, or even October 2019 according to Google Support. (Reading between the lines, that was mostly due to intransigence by device vendors, rather than by Google.)
Google now offer an RCS gateway service as a paid service to businesses, but it's still not clear when or whether there will be an on-device API.
And to complicate matters further, RCS service has only been deployed in limited cellular markets, and without having some means of testing it's going to be nigh on impossible to write the new code required to implement this feature.