Filipe Espósito, 9to5Mac:

Apple today silently removed its “Apple TV Remote” app from the App Store, which lets users control the Apple TV from an iPhone or iPad simulating a real Remote. The app is no longer available for download from the App Store and Apple has likely discontinued it, which means that it will no longer get any updates.

And:

Apple has added the Remote feature built into the Control Center in iOS 12, so Apple TV users can have access to all the controls on Siri Remote without having to download any app.

This is more a PSA than anything else. Want the Remote on your iPhone? Turn to Control Center.