I don't have a deep dive, but I can confirm youtube has made things worse. I came across a copy of 『the TV show』 that I downloaded in 2009. 720p, H.264, 2460kbps. Looking again a couple weeks ago, it was 720p, VP9, 1300kbps. VP9 is more efficient but at half the bitrate it looks much worse.

Poking more, there's an AV1 version that's a little less bad at about the same bitrate, but it's still significantly worse than the old encode. There's also a new version of H.264 that's 2/3 the size of the old one. It's about as bad as the VP9 version but the artifacts focus on different parts of the image.

You'd think after 15 years of storage prices dropping, bandwidth prices cratering, and screens getting bigger, youtube might decide to maintain bitrates and let new codecs increase the quality of videos. Or split the difference between quality and savings. But nah, worse.