Earlier quoted context omitted.
Github for some reason disables search engine indexing of wikis, not sure about discussions.
The problem that they have (and other user content created sites) if indexing was on is that SEO spammers use it to dump lots of links to sites. That you don't see this is an indication that it works and dissuading those spammers. Consider all the nearly forgotten repos out there that you occasionally stumble over and that there are no "work from home and get $30/hour - click here" or a dozen forms of "somespam.examp…
What's the difference between a spammer creating their own repo and creating a bunch of issues with spam links vs their own repo and creating a bunch of wiki pages with spam links?