Xcode 12.4 RC has announced fix.
iOS simulated devices no longer display an incorrect yellow tint in translucent UI elements. Certain GPUs may continue to display the tint with some tvOS simulators. (71203015)
I think it is a bug related to the discrete GPU.
In addition, this bug causes the background of the on-screen keyboard to be almost RGB(0,0,0). Experienced on MBP 2017 with Catalina.
Workaround I - Using integrated GPU
In the simulator, set menu File → GPU Selection → Prefer Integrated GPU to work around this bug.
Restart the simulator.
This was tested on a MacBook Pro with Catalina and Big Sur. As comments suggest that it won't work on iMac.
Workaround II - Reduce transparency
In the simulated device (e.g. iPhone), Settings → Accessibility → Display & Text Size → turn on Reduce Transparency.
Notice that it can only reduce the previous 'solid' yellow to 'fairly light'.
I find this workaround, as the discrete GPU can be used, is more suitable for testing the UI logic than the previous solution.
This was tested on a MacBook Pro with Catalina. Might work on iMac.