I'm a YouTube Red subscriber, and I also use youtube-dl to allow for offline listening on various devices. Since I'm paying Youtube, and not redistributing my downloads, I don't feel this is any different from DVR-ing a TV show or the old (and legal) practice of copying a vinyl LP to cassette tape so I could listen to it in my car.
Except the content makers aren't getting revenue since YouTube Red pays for engagement. I LOVE YouTube Red I only use youtube-dl for special use cases.
It seems likely that the gp is talking about what.cd successor redacted.ch, not the "YouTube Red" paid streaming service.
Red is great, but if we want to preserve our musical heritage, we can't need to hide as we do it. I think that secret libraries are not as resilient as public archives.