According to Twitter, these downvotes don’t do anything to the actual post or comment. On Reddit, comments with too many downvotes get hidden, but with Twitter’s approach, this is more about the company learning about what you like and what you find relevant, as opposed to the public reacting to comments.
Some of you on iOS may see different options to up or down vote on replies. We're testing this to understand the types of replies you find relevant in a convo, so we can work on ways to show more of them.
Your downvotes aren’t public, while your upvotes will be shown as likes. pic.twitter.com/hrBfrKQdcY
— Twitter Support (@TwitterSupport) July 21, 2021
The downvotes will be invisible to both the poster and public, so only you and Twitter can see it. According to Twitter, the amount of downvotes won’t change the order of replies, so once again this isn’t like Reddit or YouTube or Facebook where more popular upvoted comments rise to the top of the page.
We imagine that Twitter is trying to use this feature as a way to learn about what might give users a reason to dislike a post or comment, which perhaps further down the line could be used to further tweak their algorithm, but we’re just speculating.