Well, the thing is, that dead link is the old story, and a good number of the searches for stories don't include author names.
When they do have the author's name, most times the entire profile was deleted. So there's nothing left to leave a note on.
As to plagiarism, many of those instances do end up in the Hall of Shame. There again, though, you run into the issue with the person searching the story knowing a title or having a link but not knowing the author.
If you mean leaving the link active and changing the content, well, leaving little memo pieces like that around the archive would be "unstories" and would simply take up space in the database that we frankly don't have the room for, from what I understand. It would also be a violation of the Terms of Service and Content Guidelines, and what kind of example would it set for the users for the mods and admin to violate their own rules?
Any time there is a question, posting up the story search in the proper forum here is probably the best public way to get the information. If you don't want to question publicly, then certainly you have the option of PM'g DG or Apollo as a forum member.
Remember as well, though, that not all deletions are for those two reasons. Any violation of the ToS makes the story subject to being hidden, and if not corrected within time allowed, deleted.
And there's always the authors that choose to take down a piece. Not even staff knows when that happens until someone wonders where a story went.
The only thing we can do - if we have the URL - is verify whether it was a staff deletion. If it was an author's choice to remove their work, all we can do is let you know it wasn't a staff deletion.