Now that you can finally™ submit iPhone X screenshots of your apps to iTunes Connect, here's an easy Terminal command to take a proper screenshot:
xcrun simctl io booted screenshot "~/Desktop/Simulator Screen Shot - iPhone X - $(date +'%Y-%m-%d at %H.%M.%S').png"
This will use Xcode's built-in Simulator command line utility to save a full-frame screenshot of the iPhone X simulator (assuming it's the only one running), without cutting of the notch and rounded corners like cmd+s does. Name is formatted the same as cmd+s would do it and it's also saved to desktop.
Important thing to note: if Debug -> Optimize Rendering for Window Scale is on, screenshot might be saved at wrong resolution, so either turn that off or make sure it's at 100% scale before taking the screenshot (not "Actual Size", but 100%).