Are you running Nvidia graphics hardware? Did you update your video drivers, for instance to run Cyberpunk 2077?
Known issue with Nvidia 460.79 or later drivers. This thread is the primary thread on the issue.
https://us.forums.blizzard.com/en/wow/t/nvidia-46079-drivers/766732
TL;DR: it’s on Nvidia to fix the driver, which AFAIK they haven’t yet, but some workarounds (turn a bunch of graphics settings way down, switch to Directx 11) reduce the severity of the flickering.
That or roll the driver update back.