Are you asking when the update Nougat update is available for the S6 and pointing out it is available on other phones? If so yes Nougat is available on other devices and to date for Samsung only the S7 has it. T-Mobile cannot tell you when the update will be available as other than testing they have nothing to do with the OS updates, that is all Samsung. Google devices will always have it first since they use a "pure" version of Android. All the other manufacturers have an overlay they use which for Samsung is called Touchwiz. Touchwiz is being replaced in Nougat with what is called GraceUX which is supposed to be more streamlined and improve performance. Samsung made it clear the S7 would be the first device they did the development for with the other devices to follow. So it is all on Samsung.
T-Mobile does go one step further and give a broad status of an update and whether a device is even slated for an update, here is the link;