Why the old replicaset.apps still exist after upgrade

[houshengbo]

When we check the old resources by the label, after upgrading operator.
This is the message prompted in the end:

 =================================================:=====
==== THE FOLLOWING OBSOLETE RESOURCES STILL EXIST: ====
=================================================:=====
==== Wed Mar 11 09:06:15 PDT 2020
=================================================:=====
NAMESPACE         NAME                                          DESIRED   CURRENT   READY   AGE
knative-serving   replicaset.apps/activator-7cb88bb548          0         0         0       6m9s
knative-serving   replicaset.apps/autoscaler-6fc48c8ddd         0         0         0       6m9s
knative-serving   replicaset.apps/autoscaler-hpa-584f497bd5     0         0         0       5m55s
knative-serving   replicaset.apps/controller-8565d59f4b         0         0         0       6m8s
knative-serving   replicaset.apps/networking-istio-857f6bf6fb   0         0         0       5m52s
knative-serving   replicaset.apps/webhook-646976ffbd            0         0         0       6m7s 

Steps to reproduce:

1. Install an older version of knative serving: e.g. https://github.com/knative/serving/releases/download/v0.12.1/serving.yaml.
2. Wait for your pods up and running.
3. Upgrade to late version of serving: e.g. https://github.com/knative/serving/releases/download/v0.13.0/serving.yaml.
4. Check your replicaset.apps. You will see the replicaset.apps with the label of the older version still exist, with 0s.

Is this a correct behavior or a bug of kube?

I wait for a long time: the older replicaset.apps did not go away.