As an admin, not only do I see all deleted documents as part of a search (which mat be kind of useful) - but I also see deleted documents when I try to @ mention the name of the document that replaces it with the same name. When I bring up a document as part of a search, I'm able to view the document and see that it's deleted. But when selecting one from the list of @ mention choices, there's no way to identify which is active and which has been deleted.
While the @mention feature utilizes search results to display content, reviewing through your description, it appears there is some room for improvement when using this feature with an admin account (such as an indication of a deleted item). As such I would suggest submitting an idea to the Ideas for Jive space a next step where our Product team reviews through top rated ideas when considering features for future releases.