It turns out YouTube did't really shutdown " https://www.youtube.com/tv ". Now it adds a user agent check and only allows supported devices to continue using its Leanback interface. SmartYouTubeTV already has user agent spoofing built in. So you don't really need to do anything. But if you are curious, you can try spoofing in a browser.

Without spoofing: https://postimg.cc/y3jXGwp0

User agent spoofing as Samsung Tizen: https://postimg.cc/WhWbNXQ8

With spoofing: https://postimg.cc/k6sPj5Cs

More information:

https://github.com/yuliskov/SmartYouTubeTV/issues/340

https://www.howtogeek.com/113439/how-to-change-your-browsers-user-agent-without-installing-any-extensions/

https://developers.whatismybrowser.com/useragents/explore/operating_platform/smart-tv/