Is it possible, out of box, to enable Discovery to update the operational status of retired CIs to "Operational" status if they get discovered by the discovery process again?
Unfortunately, there is no OOB script or system property that will enable discovery to update and "un-retire" the operational_status field for a CI. Developing a customised business rule as you have done will be an option here, though, it will be outside of support for the Technical Support team.
If this is a feature that you believe should be developed, we recommend that you create an enhancement request via HI:
Self-Service > Enhancement Requests > Open an Enhancement Request
You may find the following document and it's related topics about CMDB CI Lifecycle Management useful: