GitHub currently has a robots.txt which is preventing crawling of the paths associated with the Wiki area for each and every repository. This is explicit and looks very intentional. I've asked about this (19-Oct-2019) and got no response, ticket number is 430217.

I've attached the current (27-Oct-2019) robots.txt file.

github.com.robots.20191027.txt

The gist of it:

Allow: /*/*/tree/master
Allow: /*/*/blob/master
...
Disallow: /*/*/wiki/*/*

I would like this to change to make the Wiki areas searchable using popular search engines.