iOS 12.4 is missing in Xcode 11 GM

iOS 12.4 simulator files can be found in the Xcode 10.3 app bundle. While you can't download it from within Xcode (at the moment), you can simply copy them to the folder where additional simulators are usually installed. Go to the following subfolder, it is located inside Xcode.app (10.3):

Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes

and copy the iOS.simruntime package to here:

/Library/Developer/CoreSimulator/Profiles/Runtimes

Rename iOS.simruntime to iOS 12.4.simruntime.

Restart Xcode 11.

The 12.4 simulator is now available (in Xcode 11.1 at least):