Users are being shown the following message before watching a clip...
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).
YouTube is likely making this change so that it only needs to support the App Store version of the app. The App Store is available on the Apple TV HD and Apple TV 4K.
We'll let you know when the change goes into effect. Please download the iClarified app or follow iClarified on Twitter, Facebook, YouTube, and RSS for updates.