Xcode 10.3 does not work on MacOS Big Sur 11.0.1 (non beta)
I had the same problem, I initially solved it using
defaults write com.apple.dt.Xcode DVTDisableMainThreadChecker 1. After this, Xcode 10 seemed to work on macOS 11, and everything would build from within Xcode.
I then had the weirdest issue - when compiling from the command line using
xcodebuild, the whole machine would grind to a halt and eventually hang, requiring a "hold down the power button" restart. The problem turned out to be that
ibtool (used for compiling IB files) was not honouring
DVTDisableMainThreadChecker, and was still crashing. This was somehow causing the machine to get into a state where it stopped responding.
The solution was to copy libMainThreadChecker.dylib from Xcode 12 into the Xcode 10 package. After that the problem went away.
Ok I managed to get it up and running by deleting ~/Downloads/Xcode_10.3.app/Contents/Developer/usr/lib/libMainThreadChecker.dylib It compiles my project.
To be safe, I have copied this same dylib file from latest Xcode 12.2 and put into that folder. It is still working so far.