On the page called "Understand the Kubernetes version lifecycle on EKS" - https://docs.aws.amazon.com/eks/latest/userguide/kubernetes-versions.html
The documentation states that Extended support is enabled by default, but when creating a new cluster from the console this no longer appears to be true. Standard mode now seems to be the default for both quick config and custom clusters.
I've yet to test from the CLI, but if this behaviour has changed it probably just needs to be updated to reflect that.