I know this is an old post. Stumbled upon this same issue just yesterday when on my Xcode 13 app was working fine and tested in iPhone 11 Pro for the last few weeks. Maybe was sleepy or hit some kind of wrong key but afterward only iPad devices and no iPhone devices available in simulator. Tried to close Xcode and restart the Mac with no luck. Other previous worked on iOS projects when open up still have the iPhone simulators. Tried many of the other suggestions in this comment and from Google and nothing work. What work is that I remember in the top level project folder under the General Tab --> Deployment, there is a checkbox for iPhone, iPad. Somehow the iPhone was unchecked. After checking iPhone, the iPhone simulators were available again. Hopefully this help others that have the same issue.

I had the same issue where I have multiple versions of Xcode to practice along with different Xcode books. Xcode 10.0, 11.3, and 12 all worked fine after downloading. This 10.2.1 version kept giving me the crash error and I needed this version to run some code from a different book. Tried replacing the libMainThreadChecker.dylib from 10.0 and 11.3 and it didn't work for me. The file from Xcode 12 worked so now Xcode 10.2.1 works for me. Hopefully this helps anyone with similar issue. I am very worry personally in running any terminal commands in case fixing the 10.2.1 would break my other Xcode versions. Would suggest to make a backup with Time Machine or Super Duper before making changes like this.