Carrier versions of phones tend to get updates slower than their unlocked counterparts, so it's no surprise that the T-Mobile OnePlus 6T is receiving updates later than the standard model. This time around, the update's sole highlight is a new power button shortcut for Google Assistant, with everything else merely being upgrades and optimizations.
You can read the changelog above to get the full gist, but basically everything is simply "updated," "fixed," "optimized," or "improved." The only legitimate addition is a new option in the "Buttons & gestures" section, which allows you to hold the power button for half a second to activate whatever Assistant app you have set as default (probably Google Assistant).
The update weighs in at a hair over 110MB and carries the version number A6013_45_181121. If you have a T-Mo 6T and haven't yet received this, you may want to check for updates manually.