Apple has sent an email encouraging mobile development firms to use some of the innovative new features in iOS 11, including "Core ML, ARKit, new camera APIs, new SiriKit domains, Apple Music integration, drag and drop for iPad, and more." It also announced, "Starting April 2018, all new iOS apps submitted to the App Store must be built with the iOS 11 SDK, included in Xcode 9 or later. All new apps for iPhone, including universal apps, must support the Super Retina display of the iPhone X."
It's not entirely clear what "support the Super Retina display" means. It could just mean that apps need to support its aspect ratio and resolution for the iPhone X, or it could mean that apps must be formatted to handle the notch at the top of the screen.