The Stage Manager option is available on iPadOS 16.4, however it disappeared after iPadOS 17. How to use Stage Manager on iPad Simulator after iPadOS 17?
The widely known way is(found on Apple forums):
xcrun simctl spawn booted defaults write -g SBChamoisWindowingEnabled -bool true
It's useful on newly-created simulators. However, I found it doesn't work on my simulator created months ago. After exploring the simulator config files, I found it's because the SBChamoisWindowingEnabled is YES in .GlobalPreferences.plist, but NO in com.apple.springboard.plist. So the answer would be:
xcrun simctl spawn booted defaults write -g SBChamoisWindowingEnabled -bool true
xcrun simctl spawn booted defaults write com.apple.springboard SBChamoisWindowingEnabled -bool true
It will work instantly without restarting the simulator!