Youtube always kept downvotes and the 'dislike' button, the change (which still applies today) was that they stopped displaying the downvote count to users - the button never went away though.

Visit a youtube video today, you can still upvote and downvote with the exact same thumbs up or down, the site however only displays to you the count of upvotes. The channel owners/admins can still see the downvote count and the downvotes presumably still inform YouTube's algorithms.