@MrT10001 Thanks for the instructions! I was afraid I needed to mess with Msm from Windows or something but I wasn't aware of the flashable zips in that thread. I was running LineageOS 20 with F2FS unencrypted data and needed something to really work to revert all that fun to stock

I rebooted to bootloader, fastboot booted TWRP, formatted data and everything except vendor partition, rebooted back to bootloader and fastbooted TWRP again, adb pushed OnePlus6Oxygen_22_OTA_054_all_1908302047_fulldowngrade_f019784c78234a67.zip to /sdcard, flashed it from TWRP, rebooted to System, and was greeted with the stock boot animation and OxygenOS 9.

My host OS is Ubuntu 23.04 so I got the UBports .deb and installed it with apt, opened it up from the launcher, and chose 20.04/devel. Some stuff downloaded, I followed the UBports prompt to go to Recovery, and eventually Ubuntu Touch booted up!