T-Mobile may soon be adding RCS support for Messenger as a string of T-Mobile customers seem to be reporting that RCS is active within their Messenger app. If T-Mobile is slowly rolling this out to users then it's possible they could make an official announcement on the inclusion in the near future, but for now T-Mobile subscribers who are eager to receive such support can install Messenger as their default SMS app and see if the feature support is live for them as well.
According to some of the comments on Reddit in a discussion about RCS messaging support going live for T-Mobile users, it appears that the carrier is doing some limited testing of the support for subscribers who have T-Mobile's Digits add-on. One user points out that Digits uses the RCS protocol which would make sense as a reason why T-Mobile would be using these subscribers as a small number of testers. In the same conversation it's mentioned that RCS support in Messenger should be available soon but there is no actual time frame mentioned nor is there a direct confirmation from T-Mobile on any of this just yet.
While many carriers are already offering RCS support for text messages in some form or another at this point, all of them are generally using different protocols and standards, save for Sprint as well as Rogers in Canada, who have both committed to using Messenger as their default SMS app so that they can begin to help standardize RCS across the industry. Sprint even expanded RCS support for more phones back in December of last year, so it wouldn't be too shocking to see at least one other major carrier working on implementing RCS support through Google's text app. When T-Mobile will make this an official thing, or if they will, is unclear, but if it's already being tested it may not be too far off as has been suggested. With T-Mobile officially on board with supporting the Jibe RCS protocol, which is what Google's Messenger is using, then other carriers like AT&T and Verizon Wireless may move to do the same thing, although whether or not that happens would have to wait and see if T-Mobile does indeed roll out this change.