https://github.blog/2020-05-20-three-bugs-in-the-go-mysql-dr...
> Although GitHub.com is still a Rails monolith, over the past few years we’ve begun the process of extracting critical functionality from our main application, by rewriting some of the code in Go—mostly addressing the pieces that need to run faster and more reliably than what we can accomplish with Ruby.
In 2018 GitHub got acquired for 7.5 billion dollars. Rails got them to that point and they've been up and running since 2008. I'd say that's a very big success.
Given GitHub's contributions to Rails master and other activity around projects like https://github.com/github/view_component, from the outside it looks like they're still very happy with it. It's hard to say if we'd ever get a real answer on what they think internally, it would be pretty unlikely that their CTO is going to publicly write an official company blog post on "I wish we didn't choose Rails".