Looks like GHA was announced[1] around the same time as the acquisition by MS. The first commit in that repo is a year later when they opensourced it, so we can't see how it evolved before then.
For anyone who was at Github at the time, was it always written in C# or rewritten/replaced after acquisition? If "GitHub Actions is based on Visual Studio Team Foundation Server's CI" is the case, then it sounds like the latter.
I don't recall the exact context anymore but during the Microsoft Build this year it was said by an employee at Github that the service came with the acquisition which is the reason it was and stays C#.
For anyone who was at Github at the time, was it always written in C# or rewritten/replaced after acquisition? If "GitHub Actions is based on Visual Studio Team Foundation Server's CI" is the case, then it sounds like the latter.
[1]: https://github.blog/2018-10-16-future-of-software/