In the past, yes k3s did exclude all those legacy, alpha, non-default
features from Kubernetes source as you can see in
https://github.com/rancher/k3s/pull/113. But not anymore.

As you can see in the source code, most of legacy/alpha/non-default
features is available in k3s, and misleading README should be updated.

But please, don't get me wrong. I'm actually glad to see those features
enabled in k3s. :)

Reference:
  https://github.com/kubernetes/kubernetes/compare/v1.16.3...rancher:v1.16.3-k3s.1