An unlicensed or inactive OneDrive is not automatically disposable. It can contain project evidence, personal working files required for a case, or content subject to retention. “Orphaned” should be a conclusion from evidence, not a label created by a storage report.
Large files are visible. Retention side effects often are not. That is why a storage incident can become a compliance incident when somebody responds by changing a policy before understanding what is preserving the content.
When SharePoint or OneDrive content is retained, Microsoft can preserve the original version in the Preservation Hold library after a user edits or deletes it. The user may see a file disappear while the tenant still retains a copy. Microsoft explains the workload-specific behaviour in retention for SharePoint and OneDrive.