Take this with a grain of salt, but in one of Devon Nash's videos on YouTube, he mentioned (without a source) that YouTube did a study that looked at likes and dislikes. They tweaked the algorithm based on videos that were heavily liked or disliked, and what they found is that likes and dislikes don't matter (for them). It's all about engagement.
So... People will hate watch someone they don't like, and comment on the video about how stupid that person is. These videos with high engagement end up serving lots of ads, and YouTube makes money either way. So my tinfoil hat is that when YouTube recommends a video, before if I saw the heavy dislike bar I might think, "Why is YouTube recommending this bad video to me?" But with dislikes private, I might just engage with the video and think less about why the algorithm recommended it to me.
The problem with promoting/ranking disliked content is that it may be disliked for valid reasons, like low production quality or annoying elements like bad music etc...
I think that when it comes to engagement, Youtube should just police content fairly, apply countermeasures to prevent gaming the system, and allow valid/trusted user accounts to have merit in rating content. There is no easy answer ultimately.
So... People will hate watch someone they don't like, and comment on the video about how stupid that person is. These videos with high engagement end up serving lots of ads, and YouTube makes money either way. So my tinfoil hat is that when YouTube recommends a video, before if I saw the heavy dislike bar I might think, "Why is YouTube recommending this bad video to me?" But with dislikes private, I might just engage with the video and think less about why the algorithm recommended it to me.