Knowing that it's from a few years ago gives helpful information to the potential reader.
1) Someone may have read the essay before, remembered the point, but forgotten the exact title. It has, after all, been posted on HN twice before. Simply reading the comment that it's from 4 years ago may be enough to confirm that it's the same article, as it did with me.
While the essay has the date right at the top, in general there is no consistency about where people post a date, or if there is one at all.
I know that I sometimes skim the HN comments first rather than follow a page link because then I don't need to worry about cookie notices, requests to subscribe to a newsletter, etc. This essay does not have those, but enough do that it's a general nuisance.
2) Software changes over time. Cap'n Proto's Schema language may have changed since then - perhaps as a response to this essay - making this essay of historic interest only. While that is not likely the case, it is an example of how having the date may provide helpful information.
This is useful enough that it is accepted practice to add the date to older information. For example, the previous two HN submissions to this essay both include (2019) in the title. See https://news.ycombinator.com/from?site=zenhack.net
The submission guidelines contain no such allowance. They specify a very limited set of cases where the title of a submission can be modified from the web page title.
You can see dang, one of the moderators, write: "It's the convention on HN to add the year to a title when the article isn't from this year." at https://news.ycombinator.com/item?id=27867435 ,
In retrospect, my original comment was probably too terse. It's not obvious if I was suggesting that "(2019)" be added to the title, or if I was claiming that the article was outdated for some reason. That's my fault.
