Doing a search for reactions returns a few promising results:
- Retort - a reaction-style plugin for Discourse
- ReplyGif: Adding reaction GIF easily
- Reaction Emoticons
- https://meta.discourse.org/t/is-it-possible-to-add-other-like-buttons/45252/3
That first link in particular is about Retort, a plugin that allows users to react to posts using emojis.