Some of knative webhooks (e.g. mutatingwebhookconfiguration webhook.serving.knative.dev ) has

  namespaceSelector:
    matchExpressions:
    - key: webhooks.knative.dev/exclude
      operator: DoesNotExist
    - key: control-plane
      operator: DoesNotExist

which explicitly excludes namespaces labelled with "control-plane"

The original purpose is apparently a hack for Azure AKS , see https://github.com/knative/pkg/blob/master/webhook/resourcesemantics/validation/reconcile_config.go#L153-L156 , https://github.com/knative/pkg/issues/1590 )

This causes issues with kubeflow interoperability, which uses their own "control-plane" labels.