WhenScaled specifies what happens to PVCs created from StatefulSet VolumeClaimTemplates when the StatefulSet is scaled down.
The default policy of Retain causes PVCs to not be affected by a scaledown.
The Delete policy causes the associated PVCs for any excess pods above the replica count to be deleted.
StatefulSetPersistentVolumeClaimRetentionPolicy describes the policy used for PVCs created from the StatefulSet VolumeClaimTemplates.