The old node-role.kubernetes.io/master label and taint key has been deprecated and will be replaced with node-role.kubernetes.io/control-plane instead, they are both valid during a transition period. Adding them both ensures backward compatibility while also supporting tools using the newer terminology.

The reason for the name change is that The Kubernetes project is moving away from wording that is considered offensive. A new working group WG Naming has been created to track this work, and the word master was declared as offensive and the Recommendation: master -> control plane has been accepted:

Within the Kubernetes codebase, the term “master” is often used in reference to the kubernetes control plane, either as a whole or to some subset of the components within. We recommend control plane to refer to the set of components as a whole. We recommend context-specific alternatives when talking about individual components or the roles they serve.

As part of the Kubernetes eco-system, kubeadm complies with this recommendation, more information in KEP-2067: Rename the kubeadm "master" label and taint:

Kubeadm applies a "node-role" label to its control plane Nodes. Currently this label key is node-role.kubernetes.io/master and it should be renamed to node-role.kubernetes.io/control-plane. Kubeadm also uses the same "node-role" as key for a taint it applies on control plane Nodes. This taint key should also be renamed to "node-role.kubernetes.io/control-plane".

This is also mentioned in the Kubernetes v1.20.0 Release notes