The recently released OnePlus 5T now has an official version of the TWRP custom recovery, thanks to XDA Developers user simonsickle. The TWRP version on offer is currently 3.2.0, but simonsickle has stated intent to keep it updated going forward. This version will work on any OnePlus 5T that's had the OEM Unlock option checked in the Developer Settings, and can be flashed by simply using the fastboot command to flash a recovery image. Once your device has TWRP on it and booted up, everything should work as it would on any other device. simonsickle has made the full source for this recovery available, including a device tree, kernel, a patch for file-based encryption, and a manifest.
As with anything that involves tinkering with your device's system files, there are a few caveats with this particular port of TWRP that need to be kept in mind. For starters, this release of TWRP has some issues with Android's file-based encryption, which encrypts each file with an individual key. This means that you'll have to turn encryption off before backing anything up, or trying to load from that backup will cause issues with booting. Second, booting up the stock ROM will wipe TWRP and replace it with the stock recovery, though if you use TWRP to root the stock ROM before booting up, you can always flash TWRP back on via something like FlashFire or by gaining superuser privileges and issuing flashing commands in a terminal. simonsickle did not include ZIP files for SuperSU and Magisk in the thread, but those can easily be found in their official threads on XDA.
The OnePlus 5T, for those not in the know, is a slight upgrade from the OnePlus 5. The specs are mostly the same, but this model has a redesign that includes a tall screen a la the Samsung Galaxy S8 and LG G6, and moves the fingerprint sensor to the back of the phone as a result. The other specs include the same Qualcomm Snapdragon 835 chipset that powers most flagships these days, along with either 6GB or 8GB of RAM. Alongside that, you'll find either 64GB or 128GB of internal storage. The phone is otherwise almost completely unchanged from its predecessor, the OnePlus 5, including the cameras and battery.