IMPORTANT NOTE: After checking the CRD definition of EgressFirewall for OCP v4.12 [1], it has been found that the NodeSelector is not supported in OCP v4.12.z even then also it is mentioned in the document of OCP v4.12 [2].

Using of NodeSelector in EgressFirewall CRD is only supported in OCP v4.13.z [3]. 

Please let me know if I need to open another docmentation bug for the same or this can also be worked here. 

[1] https://github.com/openshift/cluster-network-operator/blob/release-4.12/bindata/network/ovn-kubernetes/common/001-crd.yaml#L67-L79
[2] https://docs.openshift.com/container-platform/4.12/networking/ovn_kubernetes_network_provider/configuring-egress-firewall-ovn.html#configuringNodeSelector-example_configuring-egress-firewall-ovn
[3] https://github.com/openshift/cluster-network-operator/blob/release-4.13/bindata/network/ovn-kubernetes/common/001-crd.yaml#L96-L100

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