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.