If you've ever wanted to develop your iOS application(s) alongside your editor (i.e. Visual Studio Code on the left, Simulator on the right), you'll have noticed that it isn't possible on macOS by default.
In order to change this, run the following command in your terminal:
defaults write com.apple.iphonesimulator AllowFullscreenMode -bool YES
This'll allow you to use it in Split view from now on!