I’m going to make a distinction between one-line answers and “mic-drop” answers. I think there are different “rules” involved in each.
One-line answers are emergently discouraged but not against the rules: Exercise personal and collective judgement
As far as I know they’re not actively against a rule, but a lot of the site is more about process than rules. That is to say, there are things deliberately en/discouraged by how our processes work, and a hard (and brittle) rule doesn’t exist because it’s unnecessary.
Low-quality answers are one of those areas. We don’t really want them. But there’s no rule against them, because no rule could be written that would work for all cases it would need to, so we have processes that are more robust and nuanced instead.
For example, it’s theoretically possible for a one-line or one-line-plus-cite answer to be good quality. It’s unlikely, but over the course of many years the unlikely tends to come up in occasion. A rule would get in the way of getting that good answer. We’d also get people attempting to rules-lawyer such a rule, anyway.
But they’re still discouraged, so we have processes that encourage users to consider whether a given answer is low-quality enough to need action. And we have a variety of actions available (comments, editing, voting, VLQ flags, delete votes, workshopping on meta or chat, moderator-added notices, leaving as an example what not to answer, etc.), so that we can respond to a post’s individual needs for improvement or action. Sometimes a quick downvote is enough; on the far other end of the spectrum, sometimes nothing short of deletion is appropriate.
“Mic-drop” is another layer: Answers belittling the asker or question break Be Nice
“Mic-drop” answers aren’t quite 1:1 equivalent to one-line answers. At least, not the way I’ve used the term. It’s a term the mod team has used in our own discussions to mean a short answer that is written with… something… that shows disrespect to the asker. They’re often short, but not always a single line (with or without cite).
There are many ways an answer can be a “mic drop”, but the unifying element is that it conveys disdain for the question’s existence. Whether intended or not, since effect of writing is what matters and what we collectively judge with all the site’s systems.
“Mic-drop” answers are a subtle way to violate the Code of Conduct without it being blatant. They can fly under the radar, but they’re still going to hit their intended targets: askers and readers who are vulnerable to being made to feel stupid for having the question involved.
We aren’t here to make people feel stupid, and it would actively work against the site’s mission, so we have rules against that, and answers that are written so that they convey that it’s stupid to have a question are against those rules.
(If one feels like a question is stupid, it’s probably a good idea to avoid answering it. Even with the best of intentions it can leak through the writing — and when in a bad mood, it can be hard to not put it in there on purpose. Someone else will do a better job without accidentally “scoring points” on some unsuspecting stranger.)
That said, though “mic-drop” describes short, mean answers, an answer doesn’t need to be short to be mean.
All mean answers are against the rules, even if they’re not short enough to be mean because they feel like the writer just performed a mic drop to declare intellectual victory over the asker.