However, it is through @Dark Nightmare and @iaTa's efforts that we've come this far. Today, I share with everyone a new method to turn a T-Mobile 6T into an international variant. One that does NOT require an unlocked bootloader or a SIM unlock!
Before you proceed: Following the steps below WILL wipe your phone. Please backup all of your important files from the phone before proceeding.
Here are the steps:
1) Download 6T MsmDownloadTool v4.0.58 (OOS v9.0.11) provided by @iaTa
2) Download patched flasher tools here: https://drive.google.com/open?id=1dY...EBOjau1BRvtnJQ (the patches simply remove checks that allow all the images to be flashed on the phone, I've personally tested the "MsmDownloadTool V4.0_factory_patched.exe" but not the "MsmDownloadTool V4.0_factory_mcl_op1_patched.exe" variant. You can do a binary diff on the EXEs against the originals in case there's any issues with trust.
3) Shut down your T-Mobile 6T completely
4) Press and hold Volume Up and Volume down keys simultaneously
5) With those two buttons held down, plug in your USB cord to your PC
6) Ensure that the USB drivers for Qcom download mode are installed and working (available in the MSMDownloadTool thread)
7) Extract the patched tools EXEs to the same directory as where you extracted the MSMDownloadTool
8) Run "MsmDownloadTool V4.0_factory_patched.exe" (or the other one, if you're feeling adventurous)
9) Click "Start" when you're ready to start patching the phone
10) After about 5 minutes it will finish its job and reboot the phone
11) Phone will boot normally, but no more T-Mobile logo, and instead there is the OxygenOS generic logo
12) Now your T-Mobile 6T is now running identical software to the unlocked 6T sold directly by OnePlus, and running the latest software (OxygenOS 9.0.11 at the time of this writing). You can even participate in Open Betas if you'd like, and the "Local Upgrade" feature is now enabled and working.
Hope this helps somebody out.
It definitely works.