Both networking.knative.dev/visibility and serving.knative.dev/visibility are available on serverless v1.14 or before but Serving v0.21+ (serverless v1.15) can use only networking.knative.dev/visibility.
The label was completely replaced by https://github.com/knative/serving/commit/e402e8ec9acb6a5af79ebbec8d1ed11f5a7e3ff0