(The topic above was posted in the “staff” category, and isn’t visible to most users.) Auto-closing is a Discourse feature that the admins recently enabled here by default, and we are still experimenting with it. You might notice that different topics have different auto-close durations, depending on the setting when the topic was created.
The benefit of the feature is that replies to old topics account for a lot of the spam and off-topic posts we get, which currently accounts for a large portion of the moderation work load. Posts that stay open forever also make it hard for old flame wars or contentious arguments to ever really die down, since a single reply is enough to bump the whole thing back to the top of everyone’s attention.
But of course we do also get useful replies to old topics, and in some cases it will be inconvenient or confusing to have to open new topics for those instead. I hope we can find an expiration time that still helps moderators while not being too inconvenient to users. In my experience, by the time a post is about six months idle, the majority of replies are either spam or would be better as a new topic. In cases where they aren’t, it shouldn’t be too hard to link the new topic to the old one, and moderators can even re-open the old topic and merge new threads into it if necessary.