The flag has been problematic and abused by users.
While perhaps its original purpose was to be able to feed
a new version of the control-plane it also made it possible
to apply modifications to the ClusterConfiguration object
in the cluster. The lack of a feature in kubeadm for reconfiguration
of running clusters resulted in users using this flag for
the same purpose.
While it works for certain scenarios like updating
a static Pod for this control-plane only, it can result in
unexpected behavior if the user has for example fed a node name
different than the host name, when originally they created this node.
kubeadm 1.16 introduced the "kustomize" feature that
is a potential replacement for this user demand.
Add warning that this flag should not be used.
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