Missing Simulator in Xcode 8.3.2 after installing Xcode 9.0 Beta
You can select Xcode > Open Developer Tool > Simulator and after simulator simulator loads, restart and then you will get whole list back.
Note : You can not work on both Xcode with all simulator same time. You have to close one Xcode with it's simulator then you can work on another.
Restart Xcode after simulator loading process is done. You will get whole list of Simulators.
Check your deployment target in the deployment info.
This might have been changed if you are using latest version of Xcode where your deployment target will be automatically changed to higher versions of OS.