Apple just discontinued and removed the TV Remote app from the App Store with the latest iOS update. But don't panic. If you're somebody who had been using the TV Remote app on your iPhone, you can still control your Apple TV with iPhone — it's just a little bit different now.
That's because the Remote feature is integrated into the Control Center or all iPhones (running iOS 12 or later). You can access the Control Center by swiping down from the upper-right corner of your iPhone (this applies to any iPhone with a notch). If you have an older iPhone or one with Home Button (meaning no notch), you can access the Control Center by swiping up from the bottom of the screen.
Once in the Control Center, then you'll see the icon for Apple TV Remote. If you don't see it, you can turn it on by going into Settings, then Control Center and enabling it if you already turned it off. The good news is that it works exactly the same as the TV Remote app, so there shouldn't be a learning curve. You just have to remember that you look for this remote control feature in a different place.
Of course, you can still use the Siri Remote to control your Apple TV. There's no downside to this, but the Siri Remote continues to be one of Apple's most polarizing devices ever. Some people love it. Some people strongly dislike it because of small size, confusingly symmetrical design and propensity to lose itself between your couch's cushions.