I think the limit keeps posts more meaningful and “forum-like”. I don’t really get the point of single word replies. In my opinion the replies should have at least some content and be meaningful for a longer time. We already have Discord where you can spam one-liners and emojis.
But at the end of the day, it’s all about how the users want the forum to be like. If the limit isn’t good, it can be edited or removed.
Just replying “yes” doesn’t make a lot of sense, either. Why not make a poll if the point of the topic is to ask a yes/no question? Maybe you want to quote someone and clarify it with a “yes”, but doesn’t the quote count towards the 20 character minimum?