Looks like HTC One M9 units on T-Mobile will start getting the Nougat update soon. A quick look at the device's listing on the magenta carrier's website reveals that the update is currently under testing.
Sadly, there's currently no information on exactly when the roll out would begin. In case you missed, the unlocked units of the phone started getting the update in the US back in December last year. Europe got the update early last month.