Thanks for the reply, Clues. You could try
Root Oneplus 5&5T running Oxygen OS 10 based on Android 10 (YouTube) as comments there seem to indicate those instructions work.
I for my part had soft-bricked my phone by installing two different versions of Magisk at the same time (I was tired) and uninstalling them both. On the next restart all I got was a black screen and a white LED (top left) which I was able to fix
with the instructions given here, i.e.
- booted into TWRP, doing a full wipe of Cache, Data, Internal and System
- copied an OTA zip on it, I chose
9.0.11 to see if I could root that, or actually I think it was
9.0.3 that I used
- rebooted TWRP, mounted system, went to Install and proceeded with that OTA
With the phone thus unbricked, I was then able to root as per the instructions in this thread and perform the EFS backup.
I will stay under OOS 9 for a while as I've moved WhatsApp over to that phone and will see if I can extract my WhatsApp history from there. But once that's done I'll give the YouTube instructions on rooting OOS 10 on the OP5 a chance and will report back on here. If you get to try that earlier, please do let us know if it worked out.