Hi, disabling domain separation could have considerable impact to your current operational state, functionality, security, user views, reporting..... etc and I would think that you would need to test this very carefully, it may also require considerable change\rework in your platform configuration.
I think that you would need to log a request with ServiceNow Support to disable in a sub-prod instance for testing and would recommend you validate the impact of domain separation roll-back on a freshly cloned sup-production instance with a total blackout on all development that is not directly related to disabling domain separation.
I'm a little curious as to how domain separation can no longer be a requirement, or was it deployed unnecessarily?
Can you share your business drivers and use case?
