Why version at all? They went 10 years without versioning. The reasons stated for needing versioning are weak at best ("like deleting a response field, making an optional parameter required, or deleting an endpoint entirely"). Just retain backwards compatibility. Once you have versioning, people feel more comfortable making incompatible updates which is a terrible direction.
I think that's an interesting question here. Especially with how much was invested into GitHub's GraphQL and the once implication that the GraphQL API would entirely replace the REST API. (And the GraphQL API still having its own, different versioning scheme now and after the change mentioned in this blog post.)
The blog post suggests that we might find out some of the API changes that warranted this change as early as next month, but it would have been great for this blog post to include some examples of changes that were coming.
