How to run Xcode 11.4 on macOS Mojave 10.14.6

Here's an alternative solution that doesn't require downloading files from an untrusted source:

1. Install the latest release of Xcode that runs on Mojave (currently Xcode 11.3.1). It can be downloaded from Apple's developer pages.

2. From the same page, download a release of Xcode that supports iOS 13.4 (for example Xcode 11.4.1). Unzip it, rename it to "Xcode-11.4.1.app" and move it to the Applications folder, but don't run it.

3. Add a symbolic link for iOS 13.4 from the DeviceSupport folder in Xcode 11.4.1 to Xcode 11.3.1, as described in Peter Steinberger's Gist.

If you use the versions used in this answer, the command to add the symbolic link would be

sudo ln -s /Applications/Xcode-11.4.1.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/13.4 /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport

You cannot run Xcode 11.4 on Mojave but you can add support for iOS 13.4

• Download the 13.4 image from this repo
• Quit Xcode
• Unzip the archive and move the folder into /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport
• Launch Xcode

Consider that this way adds only the support for iOS 13.4, nothing else.

Update: This does not work with iOS 14 because iOS 14 requires Xcode 12.