Please read my flashing guide for Sailfish OS carefully, it highlights all the important steps and how I usually do things.
But yes, for now you need TWRP 3.2.3 AND OOS 5.1.7 firmware with LineageOS Oreo 15.1 because I haven't rebased on 16.0 yet. Here's a progress screenshot tho, I'm getting there slowly

It's essentially the same process as for the regular OnePlus 5 non-T (cheeseburger), but you just use zips specifically crafted for the OnePlus 5T with dumpling in their filenames.
To get back to Pie ROMs after testing Sailfish OS you can just flash OOS 9.0.8 firmware and a newer TWRP such as 3.3.1 from here followed by a device reboot.
Happy testing! :fingers-crossed:
---------- Post added at 11:03 ---------- Previous post was at 10:50 ----------
Holy crap, that's awesome considering I literally couldn't test if the OS even booted anywhere myself :laugh: If possible could you provide some screenshots of Sailfish OS on the 5T and send them as PM to me? You can simply take a screenshot by holding down Vol + & Vol - at the same time for a bit until the notification appears (they get placed in /home/nemo/Pictures/Screenshots/*.png in SFOS rootfs). To transfer the images your computer I recommend SSH unless you want to boot to TWRP and manually grab them from the SFOS rootfs (/ is actually in /data/.stowaways/sailfishos/).