You can, however, find USB Restricted Mode under the Touch ID & Passcode section of the Settings app as "USB Accessories".
The accompanying explanation gives no indication of its real objective: "Unlock iPhone to allow USB accessories to connect when it has been more than hour since your iPhone was locked", it says underneath a sliding bar. Interestingly, as per the researchers, authorities and private companies don't need any specific USB accessory to reset the counter.
As a concession to user convenience, the restrictions on the Lightning port only comes into effect if the Lightning port is not now being used, and the device has not been unlocked for more than a hour. It also doesn't work if USB Restricted Mode has already been triggered (past the one-hour mark). The trick, then, is to attach a USB accessory to the port as soon as possible after stealing (or seizing) the iPhone, in order to stop it from locking down. This means if you haven't turned the USB Accessories toggle on, and it has been more than an hour since your Apple device was locked, a USB accessory won't be able to communicate with your device. The resetting of the built-in feature works even with an untrusted USB accessory, one that has never been paired with the device before.
In iOS 11.4.1, and also in the current iOS 12 beta, if the Lightning port isn't used for an hour, it is deactivated. "According to our tests, this effectively disables USB Restricted Mode countdown timer, and allows safely transporting the seized device to the lab", claims Afonin.
It seems as though every new iOS updates brings various complaints from users ranging from performance setbacks to battery life issues, and iOS 11.4 is no exception. Elcomsoft is a company that makes forensic tools for governments and law enforcement, so it is in the company's interest to break this new lock.