As of OpenShift 3.6, now deployed for OpenShift Dedicated, attempting to provision a ReadWriteMany PV will fail with the following message:

Failed to provision volume with StorageClass "gp2": invalid AccessModes [ReadWriteMany]: only AccessModes [ReadWriteOnce] are supported

as per OpenShift Dedicated Restrictions.

As a result the drainer cannot attempt shared lock access defeating it's mechanism to discover that a broker pod has gone away and that a drainer pod needs to be spun up to migrate the messages to an active broker pod. This effectively breaks the ability to scale down.