SEO, click-through rate et cetera
The file from January 2013 appears just as permissive:
http://web.archive.org/web/20130109093744/http://news.ycombi...
https://news.ycombinator.com/robots.txt
shows no such thing...
User-Agent: *
Disallow: /x?
Disallow: /vote?
Disallow: /reply?
Disallow: /submitted?
Disallow: /submitlink?
Disallow: /threads?
Crawl-delay: 30
Links in comments are marked nofollow but submissions are not.