this is most likely a Google server-side update. TMobile supports RCS and I for one on an unlocked Motorola MOTO X4 have the new Android messages app stock however I only have the Dark theme with no chat options as of yet.
Not true, we're all actually waiting for T-mobile. Google already support RCS within the app but need the Universal Profile to work, which can only be activated by the carriers. At this moment only a few devices have Unversial Profile https://twitter.com/TMobileHelp/status/1012730084317265920. The supported device tmobile pushed out is 2 years old, while everyone else is waiting...
Please push out the Universal Profile to more devices! Geeee!!tmo_will