@rbhawk I managed to install then stock Android/Oxygen 9.0.8 with MSM Download in EDL mode (lowest hardware recovery mode) with this tutorial : https://www.thecustomdroid.com/oneplus-6-6t-unbrick-guide/
To start the 6 or 6T in EDL mode, set off the device and plug the USB while pressing volume + and -
The only caveat is... MSM Download runs on Windows.
Hi,
I tried to install ubtouch on Oneplus 6 with Android 10 (a mistake, I should have downgraded first to Android 9...).
Now I can just boot in fastmode (recovery mode brings me to fastboot too) and I don't know what to do to restore Android 10 or 9 - the phone is bricked... (I can't even set it off: it boots back to fastmode).
It was a brand new device I just bought to use ubtouch (enchilada) on 
I just reflashed the device with Android 9.0.8 with MSM Download, so Android 9 is running OK.
Switched to developper mode, unlocked, and then ubports-installer (0.8.8) says "Your device sdm845 is not supported".
On this page https://devices.ubuntu-touch.io/device/enchilada/ it's said this device sdm845 is supported, so what???
@lakotaubp OK, I managed to configure Android 9 and start ubports-installer.
The new problem is: ubports-installer recognize the phone, downloads files, and the 9/12 file has bad checksum and the whole thing fails:
error: download error: Error: Error: Checksum mismatch on file /home/reynald/.cache/ubports/enchilada/Ubuntu Touch/boot-caf5011e14b7d983ea9f2384ef2fc43c4937bbd0006b30ff1aa65c87ae2ef8ee.tar.xz
I will make a new post for this issue