Kubernetes is deprecating Docker as a container runtime after v1.20.
At v1.20, you will get a deprecation warning for Docker. When Docker runtime support is removed in a future release (currently planned for the 1.23 release in late 2021) of Kubernetes it will no longer be supported and you will need to switch to one of the other compliant container runtimes
What is the strategy for Rancher to maintain exising clusters deployed with rancher/rke ?
Rancher allow to deploy “Custom cluster” with rke. Did you provide an alternative with another container runtime