Microsoft Knowledge Base Article 230306 contains the following summary:
Typically, when the last domain controller for a domain is demoted, the administrator selects the This server is the last domain controller in the domain option in the DCPromo tool, which removes the domain meta-data from Active Directory. This article describes how to remove domain meta-data from Active Directory if this procedure is not used or if or all domain controllers are taken offline but not demoted first. CAUTION: The administrator must verify that replication has occurred since the demotion of the last domain controller before manually removing the domain meta-data. Using the NTDSUTIL tool improperly can result in partial or complete loss of Active Directory functionality.