If adding the year to every post that's not "news" is to become a convention, it should be added to the guidelines. I doubt this is going to happen because, really, what difference does it make for an article like this?
Old posts tend to contain outdated information, although I don't think that's the case here. The real issue is that almost every old post has already been discussed on HN previously, and posting it again just wastes time by restarting the discussion from scratch.
On articles more than a year old, I find the year helps me know at a glance if a familiar sounding title is something I've read before or fresh content with the coincidentally familiar headline.
