I’ve integrated the Discourse solved plugin which lets us mark topics as “solved”, Stack-Overflow style. If you are a moderator or the original poster and a topic is answered in a definitive way you can mark the answer as “solved” from the menu at the bottom of the reply – it now has a ‘check mark’ icon which toggles the solved state for a post.
It’s currently enabled in the Coding, tools and pipelines category only – just reply to this thread if you need it enabled somewhere else. I didn’t do any more because of the likelihood that we’re going to reorg the categories shortly.
Hat tip to @bob.w for finding the plugin