It is documented in https://access.redhat.com/documentation/de-de/openshift_container_platform/4.14/html/networking/cluster-network-operator:
"You can only change the configuration for your cluster network plugin during cluster installation, except for the gatewayConfig field that can be changed at runtime as a postinstallation activity."

Unfortunately this does not work at all and forwarding keeps restricted until you add a machine config to change the behaviour via sysctl.

always

    1. install 4.14 cluster
    2. oc edit network cluster
    3. add to spec:
  defaultNetwork:
    type: OVNKubernetes
    ovnKubernetesConfig:
      gatewayConfig:
        ipForwarding: Global
    4. save the editings
    5. look at your edits via "oc get network cluster -o yaml"
    6. look at nodes forwarding

(1) "oc get network cluster -o yaml" does not reflect the above changes
(2) nodes remain in restricted forwarding state, just forwarding kubernetes relates traffic via br-ex.

(1) "oc get network cluster -o yaml" reflect the above changes
(2) nodes forward all traffic between interfaces.