The claim that Google secretly wants YouTube downloaders to work doesn't hold up. Their focus is on delivering videos across a vast range of devices without breaking playback(and even that is blurring[0]), not enabling downloads.
If you dive into the yt-dlp source code, you see the insane complexity of calculations needed to download a video. There is code to handle nsig checks, internal YouTube API quirks, and constant obfuscation that makes it a nightmare(and the maintainers heroes) to keep up. Google frequently rejects download attempts, blocks certain devices or access methods, and breaks techniques that yt-dlp relies on.
Half the battle is working around attempts by Google to make ads unblockable, and the other half is working around their attempts to shut down downloaders. The idea of a "gray market ecosystem" they tacitly approve ignores how aggressively they tweak their systems to make downloading as unreliable as possible. If Google wanted downloaders to thrive, they wouldn't make developers jump through these hoops. Just look at the yt-dlp issue tracker overflowing with reports of broken functionality. There are no secret nods, handshakes, or other winks, as Google begins to care less and less about compatibility, the doors will close. For example, there is already a secret header used for authenticating that you are using the Google version of Chrome browser [1] [2] that will probably be expanded.
[0] Ask HN: Does anyone else notice YouTube causing 100% CPU usage and stattering?
https://news.ycombinator.com/item?id=45301499
[1] Chrome's hidden X-Browser-Validation header reverse engineered https://news.ycombinator.com/item?id=44527739
[2] https://github.com/dsekz/chrome-x-browser-validation-header