The RCS interfaces (APIs) on Android are currently only available for Google internally to use, no other third party developer has access.
We are hopeful Google will make these RCS interfaces available in the near future to all developers (in the spirit of Android open source), so that Textra SMS can also offer RCS functionality.
Shameless plug for Textra as a much, much better alternative.
No RCS (not their fault, but still).
TIL