This sounds like a useful plugin. As a core thing, that'd call for a database change that would be specific to this use case, not a general need. I see the similarity to date added, though (it sounds like a workflow-based feature, which are often specific to projects. The Omeka Curators Dashboard plugins from UCSC tackle a lot of similar issues).
Either way, though. what behavior would you expect if someone makes something public, then changes their mind and makes it not public?