I will make a final summary, there are two ways:
1. I originally provided this, reference this
This is my download path, you can download 9.0
1.1 cd ~/Library/Caches, find com.apple.dt.Xcode, show in package. Paste DMG into the Downloads folder (if not created).
1.2 Restart Xcode, open the panel Xcode->Preferences->Components, you can see iOS 9.0 Simulator, click to download
In practice, Xcode 10.3 could not be seen to see the download button of the iOS 9.0 Simulator.
It seems to confirm @Adobels Xcode supports SDK latest three versions of the iOS SDK
2. Download .simruntime and put it directly into the /Library/Developer/CoreSimulator/Profiles/Runtimes/ folder.
The Runtimes folder has all the emulators you installed, suffixed with .simruntime.
Apple did not provide the .simruntime download address, I downloaded it from the cloud disk shared by others, of course, if you installed the old version of Xcode.
I am working after the test, iOS 9.0 Simulator will open the prompt Safe Area Layout Guide before iOS 9.0, and finally close the Safe Area Layout Guide.