In November 2019, k3s graduated from being just a community project led by Rancher Labs to a fully conformant Kubernetes distribution that is supported commercially by Rancher Labs. View release notes for k3s v1.0.0 here. With the general availability of k3s, Rancher Support extends to k3s clusters.
Comprehensive Rancher Support, inclusive of Kubernetes and Docker, applies only to RKE-provisioned clusters. What this means is the following:
In the RKE-provisioned clusters, Rancher can provide patch fixes as needed at the levels of Kubernetes and Docker.
For clusters that are brought under management of the Rancher control plane, as imported clusters, Rancher Support applies only to those Kubernetes versions published in the support matrix and to the extent of making sure Rancher control plane functionality works as published in Rancher docs. Issues root-caused to be inside these clusters will need to be taken up by Customer with the provider of support for these clusters.