Youtube’s tv app is actually just a website (youtube.com/tv, although you need a tv user agent). So yeah, I think most tvs are using JavaScript and the rest are using the tvlite api which has less formats than web_safari (which will continue to work in yt-dlp without Deno if you’re willing to accept 1080p downloads with inferior codecs)
They have been using the older APIs kept around for the benefit of those smart TVs for a very long time, but things move on and newer TVs get fancier hardware and more full-featured software, which includes YouTube, and so Google has started proactively dropping support.
Also what kind of environments are executing the JS? If Google begins to employ browser fingerprinting that may become relevant.