The functionality is reportedly gated by three things: /AppleInternal folder, DraggingEnabledPhone and DraggingEnabled BOOLs in UIKit prefs plist.
"Clearly it works great, and they’re using it internally. If you want to see drag & drop on iPhone, you’re going to have to make some noise." says TroughtonSmith.
He also notes that interprocess drag and drop on iPhone is gated by the BOOL 'CancelOnDeactivationPhone' in the UIKit prefs.
Given that the feature is enabled with pref keys on the iPhone, it appears that Apple could be planning to debut this at a later time, perhaps alongside the iPhone 8.
If Apple’s gonna have an onscreen homebutton on the iPhone 8, it would make a lot of sense to spring-load it for drag & drop. A drop target
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