From what I can tell, Github transitioned over to micro-services ~2020 (https://www.infoq.com/presentations/github-rails-monolith-mi...). By this point it had already grew to a scale that very few companies will ever reach.

I'm not sure at what point the monolith codebase became absolutely enormous, but I would bet that GitHub grew to 1M active users with a monolith just fine.

Micro-services might be necessary for the companies like Github, Twitter, Google that grow to be huge, but monoliths seem to work just fine for the vast majority of other companies.