We can add the istio injection annotation by patch command:

oc patch deployments.apps -n knative-serving activator -p '{"spec":{"template":{"metadata":{"annotations":{"sidecar.istio.io/inject":"true"}}}}}'

But as an admin, we would like serverless-operator to configure it automatically. Also, the annotation is dropped when Deployment is recreated.

Please refer to https://github.com/knative/operator/issues/165 more detail.