The iOS simulator in Xcode 9 has made a curious change.
The original iOS simulator had a skeuomorphic design that looked like a photo realistic iPhone on your desktop.
Before the iPhone 5 and its tall screen were introduced, the simulator lost its chrome and shipped with a resizable window, telegraphing the introduction of new screen ratios.
I think history is repeating itself. The simulator in Xcode 9 returns to a more skeuomorphic (although not quite photo realistic) design, and an option to turn it off labelled “show device bezels”.
I suppose it could just be a change for changes’ sake, but I think they’re preparing for a new phone that doesn’t have a strictly rectangular screen, and knowing how the bezels appear will be important when developing your app.
Whether it’s just rounded at the bottom and/or top or has a notch at the top, or cut out for a speaker is anyone's guess at this point though.