Thank you for the idea. Idea visibility can be controlled at the idea level using the visibility settings. You can also set a default visibility setting for new ideas. You can find more details in the Idea visibility section of this article. We intentionally keep idea visibility separate from workflow statuses so that ideas can transition through the workflow without being removed from the portal.
Based on the options available for managing visibility we are unlikely to implement this idea at this time. We hope you understand.