This still has not been resolved as of 2/13/21. Turning shadows to low does not get rid of all the flashing. It’s especially bad in Ardenweald. I will not turn down to DX11. PLEASE get with Nvidia and get this issue fixed!
For what its worth, I had this issue today after i updated windows. It went away after updating my nvidia drivers. Check to see if you have a windows update maybe?
Did you set it back to high shadows after the updates and no flickering?
Blizzard should say that DX12 is no longer officially supported. Problem fixed. 
You should be tracking the main thread on this topic.
https://us.forums.blizzard.com/en/wow/t/main-thread-nvidia-460xx-driver-flickering/766732
Any breaking news from Blizzard will be in there.