YouTube is sending emails to Apple TV third-gen owners saying it will remove the app in the future (via 9To5Mac).
YouTube on Apple TV
Here is what the email says:
Starting early March, the YouTube app will no longer be available on Apple TV (3rd generation). You can still watch YouTube on Apple TV 4K, Apple TV HD, iPhone, or iPad. With AirPlay, you can also stream YouTube from your iOS device directly to any Apple TV (3rd generation or later).
Once it’s removed, Apple customers with this Apple TV model will have to start a video on their iPhone or iPad and use AirPlay to stream it to their TV box.