Apple has posted an update to developers of iOS that requires them to use the iOS 11 SDK and to support the display of the iPhone X. The policy change goes into effect in July and any apps submitted or app updates submitted must comply with the new policy or they will be rejected.
The requirement for using the iOS 11 SDK is not a surprise. It has been available since September of last year and Apple likes to make sure that apps, just like iPhones and iPads, are up-to-date. Part of that SDK is supporting the Super Retina Display of the iPhone X, including its notch. That is now required too.