Major mobile carriers in the US including AT&T and Sprint has already rolled out One UI update for Galaxy Note9 but still, Verizon and T-Mobile are lagging behind in this race. As an update to this matter, T-Mobile has begun testing the Android 9.0 Pie update for this particular device which really progresses from the perspective of T-Mobile.
According to the support page of T-Mobile, Galaxy Note9 is currently in testing phase for Android 9.0 Pie update. At this stage the device goes through certified testing process to find and fix various bugs and issues in the developed build.
T-Mobile's update program consist of three stages including Development, Testing and Complete (Rollout). Now when the company has put the Note9 in test stage we can expect the consumers of Note9 on T-mobile can receive the Pie update anytime soon.