Juli Clover, writing for MacRumors:
Apple today sent out a notice to developers letting them know that starting in April of 2018, all new apps submitted to the App Store must be built using the iOS 11 SDK, which is included in Xcode 9 or later.
Furthermore, Apple says that all new apps designed for the iPhone, including universal apps, must support the iPhone X's Super Retina display.
"Must be built", unfortunately, doesn't mean apps have to support new features like drag and drop. Speaking of which, I don't think supporting the native resolution of the 12.9-inch iPad Pro is a requirement yet, and the device launched in November 2015.