Setting Privacy Status to OPT_OUT will cause the V5 SDK to purge all data and identities which includes the ECID. The registration token is set in the app subscriber table after it is forwarded from the Visitor ID service. Using collectPii to set a blank token is not recommended. Do you have a use case where an opted-out user should have their push token cleared from the ACS subscriber table? If so, we recommend that you open a customer care ticket with Adobe Campaign so the work can be prioritized.