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.