Should a tag be created for "closed"/"rejected" questions? It might make sense to have them tagged if they're being kept for archival purposes—it would also mean that a question like this one on Win32 worms won't have a tag about "optimization," when the question has absolutely nothing to do with optimization. . . .
Closed questions can already be searched, by searching for "closed:1". Most closed questions are kept around. If we really want to have a "this question was closed for historical reasons" banner like Stack Overflow, we should submit a feature request.
Closed questions that are way off-topic, like the Win32 worm question, should be deleted. Thank you for reminding me to do that.