Starting July 2018, all iOS app updates submitted to the App Store must be built with the iOS 11 SDK and must support the Super Retina display of iPhone X.
Apple is very generous when it comes to enforcing new standards, such as this. I won’t pretend to know the complexities that are managing the design of an app with a significant user base, but using apps that still aren’t updated for iPhone X is a glaringly bad experience. For instance, the Roku, Ring, and Alexa apps were only recently updated to support the Super Retina Display. Up until then, I used to twinge every time I had to launch one of them.