The main web backend is still ruby it seems: https://github.blog/2020-08-25-upgrading-github-to-ruby-2-7/ I can't remember where it was exactly but I remember recently stumbling on a random github-related OSS project that had chatter in the issues from githubbers talking about .NET and C#. It would not surprise me to see significant pressure from the now 2-year post-acquisition engineering org to get in line with the rest of MS's online services that are all .NET stack.