I dunno. I lived through Internet Explorer, SCO and Linux patent lawsuit days so I reckon it's only a matter of time before Github goes ASP.Net.
A friend at GH was already writing mostly C# and Go before he moved to engineer manager 2+ years ago, so I believe it's already ongoing. Also he absolutely hated having to jump into components in RoR before Codespaces was available internally.
C# is used for Actions: https://github.com/actions/runner, and Go is used a lot for internal services. There is no traction rewriting our monolith in C#.