Supporting many API versions in parallel definitely has an overhead from an engineering perspective - but, as the blog post says, we thought it was crucial that we unlock the ability for us to evolve our API whilst still providing consistency to existing integrators. Some form of versioning was the only way we could see to do that.
We've built some pretty neat tooling and abstractions internally to make it easy for teams across GitHub to build and test multiple API versions. We hope to publish a blog post about that in the near future.
