USB Restricted Mode was first noticed by security researchers ElcomSoft back in May during the beta of iOS 11.4, when it was discovered to automatically password-lock an iPhone's or iPad's Lightning port after seven days of inactivity.
Apple curiously didn't include the security in the release notes, probably to not alert "interested parties".
Now, Apple should be testing iOS 12, watchOS 5, and tvOS 12 exclusively, as you shouldn't expect to see anymore iOS 11 updates.
The toggle is off by default, which means that once your iOS device has been locked for more than an hour, the operating system will no longer allow USB access to connect to the device.
Essentially, after one hour of non-use, an iOS 11.4.1 handheld will not talk to any device plugged into its charging port - other than to receive power to charge its battery - until a valid passcode is entered via the screen. That seems to be the case for Apple's latest controversial security feature which was initially expected for iOS 12 in a few months. You can, however, find USB Restricted Mode under the Touch ID & Passcode section of the Settings app as "USB Accessories". These are also small updates. Apple TV owners can also grab tvOS 11.4.1 today. There isn't anything in the way of new features, instead delivering on some bug fixes and stability improvements. If you have taken the opportunity to upgrade your compatible device to iOS 11.4.1 and immediately regret that decision for one reason or another, then it is possible to downgrade the firmware. Those releases are likely to come this fall.