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.
Update your version of Xcode to the latest release of Xcode 9 available on the Mac App Store, which includes the iOS 11 SDK, and build your apps. Starting April 2018, all new iOS apps submitted to the App Store must be built with the iOS 11 SDK. All new apps for iPhone, including universal apps, must support the Super Retina display of iPhone X.
With this requirement, Apple is aiming to encourage developers to adopt key features introduced in iOS 11, like Core ML, ARKit, new camera APIs, expanded SiriKit domains, and more, plus the company is making sure future apps will be fully compatible with the display of the iPhone X.
Apple will, for the time being, allow apps built using earlier SDKs to continue to be updated without switching over to the iOS 11 SDK, but at some point in the future, Apple is likely to require developers to use the iOS 11 SDK for app updates as well.
April 1, 2018 is also when Apple plans to stop accepting updates to watchOS 1 apps. All updates submitted after that date must be built using the watchOS 2 SDK or later, and all newly submitted apps must be built with the watchOS 4 SDK or later.