Apple TV3, launched in 2012, will lose its YouTube app next month. It will still be possible to watch YouTube via AirPlay from an iOS or macOS device.
YouTube on Apple TV3
In April 2015, the YouTube app stopped working on Apple TV2 (from 2010) and in March 2021 the app will stop working on Apple TV3 (from 2012). The change will not affect the more recent Apple TV boxes running tvOS.
It will still be possible to watch YouTube on Apple TV3 by using the AirPlay function on an iPhone, iPad or Mac.
- "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)," said YouTube.

Apple TV3 has a different user interface than the newer, tvOS-powered boxes
Apple stopped selling Apple TV3 several years ago but the company has continued to support the box with updates ever since, most recently with the Apple TV app (in software 7.3) and iOS control center support (in software 7.5).
It is Google's decision to discontinue the YouTube app. Likewise, in 2012, YouTube was removed on Smart TVs from the 2012 generation or earlier.
- Source: 9to5Mac