I've downloaded the Xcode update that includes an update for the Simulator app with new devices (iPhone 8, X, etc.)
It seems that with this new update, the only one of all of the devices listed in the Simulator that I can actually inspect using Safari's inspector is the iPhone X. If I attempt to inspect any other device, the 'simulator' dropdown under 'Develop' in Safari doesn't show. This also happens if I switch from the iPhone X to another device and switch back, the iPhone X is no longer inspectable.
This essentially renders the simulator uselss for web inspection.