This is related to the functionality that was introduced in OCP 4.15 brandh as per K8s upstream of cleanup_controller: https://github.com/openshift/cluster-openshift-controller-manager-operator/blob/release-4.15/pkg/operator/internalimageregistry/cleanup_controller.go#L179
It would be great to have a way to omit secrets by an annotation so specific secrets are not deleted or any other way.  

    Any version major than 4.15

    The controller will always remove it as per design as long as the registry is removed which is commonly used in customer environments.