Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
Youtube likely has never used downvotes as any sort of metric for algorithm so their removal of them is entirely about user behaviour, based on research like this. Their algorithm work will be pretty much 100% derived from what keeps people watching the most and longest and what keeps the ad clickers clicking ads. They will be completely zoned in on total activity and profit. Not quality. This is unfortunately also why outrage content and debatebro reaction drama is all the boom regardless of its obvious harm socially.