Apple's recent move in its ongoing word war with the FBI, the National Security Agency and other intelligence and law enforcement agencies over encryption is a feature that disables USB data access on locked iOS devices. 1
The functionality of the USB-restricted mode is actually very simple. Once the iPhone or iPad has been upgraded to the latest version of iOS that supports the feature, the device disables the USB data connection via the Lightning port one week after the last time the device was unlocked.
At this point it is still unclear whether the USB port is blocked if the device has not been unlocked with an access code for 7 days; if the device has not been unlocked at all (password or biometrics); or if the device has not been unlocked or connected to a trusted USB device or computer.
It's easy to see why Apple considered it appropriate to introduce the feature: authorities have rushed to buy Graykey encryption devices from Grayshift Company, which allegedly hired a former Apple technician. Graykey devices can be connected to the Lightning port of an iPhone and decrypt it over an unknown vulnerability within hours or days. This gives the authorities the long-awaited backdoor for encrypted iOS devices that Apple does not want to provide.
With this feature enabled, an investigator or someone else hoping to break into an Apple device will have a limited time frame left. Automatically shutting down the USB port also means that Apple has developed a method to prevent device access without Grayshift and its competitors, such as Cellerbrite, having learned of the vulnerability.
When USB Lockdown Mode Is Enabled, Authorities Have Gained It's not possible to simply hide iPhones that they can not unlock in Beyond Lockers until a new technology breaks in to block the disabled one Lightning Port, which somehow sounds difficult. It also means agencies like FBI will not be able to figure out how to force Apple to build a sophisticated hacking tool the next time they buy a phone they do not have in their hands Unlock period. In essence, this device is at a different level of hardware locking that Apple can not disable afterwards.
Other recent iOS 11 updates have introduced local backup time limits that may be used to extract information from a MacRumors device.