I need an index-page, that shows links to all gitHub repositories.
I think that is the reason, why many repos are not found by crawlers like the Waybackmachine
I think if there was such a site with a high ranking, they would start crawling it
The developer site sais, there is an Api for getting all repos
Warning: GitHub hosts a huge number of repositories. You'll have to take this into account when designing your index.
I can think of a few options: