The OnePlus 6T has been running Android 10 for a few months now, but not so on T-Mobile. Units bought from the magenta carrier have been stuck on Android 9 Pie, for what seemed like forever. Not anymore, though.
The carrier has finally started rolling out the update to Android 10 for its OnePlus 6T units today. As usual, this is a staged affair, so it may take a few days before your TMo 6T pops up that update notification, but it is on the way nevertheless.
Once you install the ~1.6GB new software, you will have Android 10 on board, as well as the December 2019 security patch level. Yes, that's incredibly old at this point in mid-February, and OnePlus still doesn't understand why the word "monthly" is present in "monthly security updates". That said, this is a less ancient patch level than the previous one the 6T was stuck on, so that's definitely progress.
Last but not least on the changelog, we've got our usual assortment of "general bug fixes", and let's not forget improved system stability. The update is known as A6013_34_191228.
Which is why you are better off, with a NON carrier phone. Updates, if any, will arrive sooner, than having to wait for the carrier to lock down features, or remove them along with adding their bloat.
That's weird because my unlocked 6T received the Android 10 update last Christmas and the December security patch update earlier this month.
T-Mobile is just being cheap. They don't have enough people on hand to do validation so everything got delayed by months