iPhone X UI support for existing app on Appstore
Once relevant developer news worth checking is this:


Submit Your Apps to the App Store



  Starting March 2019, all new apps and app updates for iPhone, including universal apps, will need to be built with the iOS 12 SDK and support iPhone XS Max.


So yes, you'll be required to add support for iPhone X/XS/XS Max when releasing an update for your app.

However, there is no word on if Apple will remove all the non updated apps lacking support for notch.

Based on the pattern followed so far, if your app becomes a candidate for removal from the App Store (for whatever reason), Apple will typically inform you in advance and request you to push an update to the app and give you a deadline to do so.