OnePlus 6T is another flagship killer phone by the OnePlus community. And we know that rooting this phone does not void your warranty. Moreover, rooting the OnePlus phone is an easier process amongst like other phones.
Right now TWRP is a temporary one. But you can root in this way easily until we receive the official TWRP. So in this way, all you need to root is the OnePlus 6T phone, a Type -c USB cable and a Type-C Pendrive or a Type-C SD card adapter.
On your phone first backup everything like images, videos, music, and anything that you are left in the drive. And make a back up to your pc or another phone because you are gonna lose all the data files. And then on your OnePlus 6T go to Settings then go into about phone and click on build number 7 times until you open up the developer options and then go back and click on System you will find the developer options. After opening developer options now click on OEM unlocking and give your pattern and password, you will also find USB debugging, enable that.
Now go back to your phone and connect to your PC with Type-C cable. You need to download Toolkit on your pc. Now open up the toolkit and once you open up the toolkit on your phone there will be a pop-up which will show allow USB debugging, tick that icon and click on OK and it will show green icons and it will detect the OnePlus 6T. Next, you click on unlock Bootloader icon and it will basically go into the bootloader mode and it will show a prompt on your phone. Then click on the volume button and select yes and Ok by pressing the power button. And now the phone will reboot and wipe the whole device and it will start again. So wait for it and when it's done, set up all again and then go to setting>About Phone and click the build number 7 times. Then go to the developer options again and enable USB debugging and also enable advanced reboot. Then it will again show up a pop up for USB debugging providing access, so press OK.
Next, open up the toolkit and click on recovery flasher and then select 'Manual Mode' and click on the recovery file
and open that up on flash and once you open that with flash it will basically go into bootloader mode and flash the recovery and open up the twerp recovery. Click read-only for now as this is not the official TWRP. Then copy the Magisk zip file on to the USB Type-c Pendrive or the sd card that you have. Then plug in the USB type-c onto the one plus 6T and then on the TWRP go onto the Mount>USB storage and click Ok. Now go back and click on install and click on the Magisk zip file and swipe to confirm flash. Once it's done you can go ahead and reboot your system. So, You are done!
You can see the Magisk is installed you can check with SuperSU that the root is enabled.
It's not official yet. But when it's available you can directly root your devices with Magisk.