I think I'm experiencing the same problem. Fresh MacOS, fresh MacBook (M1 processor), fresh install of Xcode (13.1)... downloaded iOS 11.3 Simulator, but I cannot seem to get it to show up anywhere. I've rebooted, but if I click 'Add Additional Simulator', and click the '+' next to the filter at the bottom of the list of Simulators, I can in no possible way get the OS Version dropdown to list iOS 11.3.