Yesterday, we reported that T-Mobile was in the process of testing Google's/Jibe's RCS implementation. T-Mobile has had its own version of RCS (called Advanced Messaging) since 2015, but like most implementations of the RCS technology, it is not compatible across carriers like SMS.
T-Mobile implementing the Jibe Universal RCS Profile, which Sprint and (soon) Rogers support, would certainly be better for users than the current Advanced Messaging system. Unfortunately, there is no substantial evidence that it is happening at this point.
Most of the initial reports came from this Reddit thread, with the OP later saying he is not a T-Mobile customer. Furthermore, a comment on the Reddit thread 'confirmed' the rumor with a poorly-censored Hangouts conversation with a 'source.' When I wrote the original post on this rumor, I disregarded those screenshots for lack of verifiability. However, I did not notice that the blurred-out name matched the Google+ user who tipped us about the rumor - essentially citing himself as the source.
In summary, there is no concrete evidence of Jibe RCS being tested on T-Mobile. We have reached out to T-Mobile for comment, and we will update the post if we get a response.