Mark --docker-exec-handler deprecated

cmd/kubelet/app/options/options.go

@@ -213,7 +213,9 @@ func (s *KubeletServer) AddFlags(fs *pflag.FlagSet) {
 	fs.MarkDeprecated("babysit-daemons", "Will be removed in a future version.")
 	fs.Int32Var(&s.MaxPods, "max-pods", s.MaxPods, "Number of Pods that can run on this Kubelet.")
 	fs.Int32Var(&s.NvidiaGPUs, "experimental-nvidia-gpus", s.NvidiaGPUs, "Number of NVIDIA GPU devices on this node. Only 0 (default) and 1 are currently supported.")
+	// TODO(#40229): Remove the docker-exec-handler flag.
 	fs.StringVar(&s.DockerExecHandlerName, "docker-exec-handler", s.DockerExecHandlerName, "Handler to use when executing a command in a container. Valid values are 'native' and 'nsenter'. Defaults to 'native'.")
+	fs.MarkDeprecated("docker-exec-handler", "this flag will be removed and only the 'native' handler will be supported in the future.")
 	fs.StringVar(&s.NonMasqueradeCIDR, "non-masquerade-cidr", s.NonMasqueradeCIDR, "Traffic to IPs outside this range will use IP masquerade.")
 	fs.StringVar(&s.PodCIDR, "pod-cidr", "", "The CIDR to use for pod IP addresses, only used in standalone mode.  In cluster mode, this is obtained from the master.")
 	fs.StringVar(&s.ResolverConfig, "resolv-conf", s.ResolverConfig, "Resolver configuration file used as the basis for the container DNS resolution configuration.")