iPhone X UI support for existing app on Appstore I have application on store which last updated 2 years back and compliant with 64 bit requirement but now there is no active development going on this   application but we don't want to disappoint existing customers who are using this application. 

But when we tested app on new iPhone X and other devices with top notch display it show black bar at top and bottom. So is there any guideline to update existing app to make it compliant with iPhone X display otherwise apple will remove from store ?
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.