The recent partnership between T-mobile and OnePlus may yield good results for OnePlus 6T users on the long run. Although the the OnePlus 6T smartphones will be locked to T-mobile and users will only use T-mobile ready SIM on the smartphone for the next two years for a discount price of $300 as 24 bill credits.
For people that may want to unlock the bootloader it is sad to say that you will only be allowed to unlock the boatloader after the next two years. But this method is too old and smartphone manufacturers did it before and have all left it.
You can also pay off the remaining of the smartphone at anytime however the bills will be added until the two year period has elapsed.