Sorry, that's not meant as an ad - it's RYD comments, which only show up on videos where YouTube's own comments are turned off. Everyone can read and react there; posting needs a Commenter membership, and that's what the link is for. "Hide RYD comments" in the settings turns the panel off.
On the browser data worry: RYD only runs on YouTube and doesn't read other websites. It sends the video ID to look up dislikes, and your likes and dislikes with a random ID.