I know that some users do choose to use "tl;dr" after their posts, and many of them given conclusions or brief introductory paragraphs that address the succinct answer and then delve into the longer specifics.

I'm not against anyone who writes a good answer who wants to add a "tl;dr" section, but I would worry that it would set a bad precedent. For the most part AskHistorians is the most anti-tl;dr forum one could imagine. The whole point of the subreddit is to give in-depth answers. It is not anathema to that purpose to tack on "tl;dr" to a post, but some people might mistake that tl;dr section as the substance of the post rather than the ultra-convenience that it is, and that could lead to a proliferation of low-quality answers.