I’ve noticed an uptick recently both in general spam, and also spam that’s getting past the automatic filters on the site. A lot of that spam targets posts that are deep in the archives, so I’ve decided to go ahead and close off commenting on older posts.
Right now the commenting threshold is set to 21 days. I think this is a reasonable amount of time to get comments in. One, the vast majority of comments on any post happen within the first few days anyway. Two, comments that arrive after the first couple of weeks tend to be by flyby cranks who are adding nothing useful in anyway (sometimes good comments get added on late. But there’s much more of the former than the latter).
Because most comments happen in the first few days, I don’t really expect this to present a major change in the site’s utility for most commenters. However, if for some reason it does, drop me a line and let me know; I’ll factor it in for any later tweaks.