In noStrudel, it looks like it's just the minimum default reaction. Even if you delete all the quick reacts, you still have the + (thumbs up) and - (thumbs down) reactions.

What's interesting is that I used the thumbs down earlier, and when examining other clients, it doesn't register as a downvote and doesn't even show up in the list of reactions. I'm not quite sure if that's just due to the client itself or if something else weird is going on.