How do you power-down an iPhone X in the Xcode simulator?

Change your simulator to required version from Hardware -> Device -> That way you will have correct version loaded and of course there is no way of "Shutdown" a simulator but quit, as this is just an application not hardware. If you have multiple simulators open, you can close the not required ones by highlighting them and select File -> Close Window or CMD+W

Another good info from Colin :

I also just noticed that if I turn off "show device bezels" I get regular window controls for closing the window, too.