The button is still there, you just don't see the number of dislikes without a plugin.

Is it? I see in the network requests where the "likes" button data is returned, but not the dislikes. It's possible I just can't find it though - there's a lot of data in a lot of shapes.

The dislike count is from an external tracker, not YouTube's own data.

That makes way more sense, but seems like it's a useless feature. I would assume that in that case it would be dislikes from the add-on (or some sort of federated dislike space?), which is really just going to tell you the opinions of people who like disliking things so much they downloaded an add-ons just to see others down-vote it as well.