There are several workarounds that users can try to download and install the latest version of YouTube on Android TV 4.4.4:
Third-party apps like SmartTube use an activation code system. Follow the on-screen instructions to log into your YouTube account via a smartphone or computer web browser. If you want to get this set up, let me know: What brand or model of Android TV box do you have? youtube for android tv 4.4 4 latest version download
Open the web browser on your TV or use a computer to download the YouTube_11.x.x.apk or SmartYouTubeTV.apk . There are several workarounds that users can try