Further to Ryano89's post #1...
The T-Mobile (USA) minority variant of the OnePlus 6T has finally received a minor update.
The update is dated the 29th October, so that gives you a rough idea of how long it takes T-Mobile to certify updates, (about 3 weeks)
Unlike the global model's update, 5th November, which comes in at 1.64GB, this is only 132.3MB.
The software update includes the following:
Android security update to Nov 2018
Fine tune Nightscape mode
In-display fingerprint algorithm update
Update Unlock app
Gesture Mode 2.0 update