Downvotes are used by google to suggest content. If you downvoted something, you engaged with it, which google loves and wants more of.

The incentive for users/viewers to downvote something has always been to give other viewers a heads up. This doesn’t work anymore

Is "downvoted count as engagement and this boost the video" a documented result or just a rumor?

People respond to feedback mechanisms. If you hid comments on videos, do you think people would be as inclined to add their own comments as much when it is literally going into the void?

Previously, disliking would increment the global "dislike" counter and users got immediate visual feedback that their dislike was counted, even if that state may take minutes for Google to reconcile and store.

Now, clicking the dislike doesn't do much and users have no indication whether others disliked the content too.