iOS 11.4 has been released to beta testers and includes USB Restricted Mode. If an iOS device hasn't been unlocked either using Face ID, Touch ID or a passcode for seven days, the Lightning port's data functions will be disabled, allowing the device to only be charged.
Security software company Elcomsoft has tested the feature and verified that if an iPhone hasn't been unlocked or connected to a paired computer in the last 7 days using a passcode that data access through the Lightning port is disabled and it is restricted to charging.
Placing a time limit on the Lightning port means law enforcement agencies will have a limited time to access any data on devices - or at least make it harder if they can't get access through one of the usual modes. And while there are cracking devices, such as the GrayKey box around, the window for using them is no longer indefinite.
Apple's developer documentation says "To improve security, for a locked iOS device to communicate with USB accessories you must connect an accessory via Lightning connector to the device while unlocked - or enter your device passcode while connected - at least once a week."
USB Restricted Mode was initially included in iOS 11.3 during its beta program but held back from the production release. So, although it's in the iOS 11.4 beta that doesn't mean it a definite inclusion for iOS 11.4. But it puts law enforcement agencies on notice that the feature is coming.