pg also commented that he doesn't want traffic from Google anyway: https://news.ycombinator.com/item?id=5808990

In which case, he should add: <meta name="googlebot" content="noindex"> to the html head of every page.

(I have to say, that's a smart way of avoiding any Eternal Septembering, but it'd be a shame. I often use Google to find old HN threads that I vaguely remember from months or years ago.)