I'd like to see it added as a special (non user selectable) tag similar to the
[status-declined] type tags here on meta.
Not only would that give an immediate visual indication,
but it would also have the extra benefit(?) of allowing you to include/exclude .
[closed] questions in searches
Edit: I forgot, but as @waiwai933 pointed out, you can already search for closed questions (
closed:1)... however I still think the visual indication alone makes it worth implementing.
If you wanted to take the idea further though, you could expand the
[closed] tag to different tags for each close reason (i.e.
[migrated-to-sf]) then you'd get some potentially useful search abilities since you'd be able to find questions closed for specific reasons... (plus you could find all questions migrated
/questions/tagged/migrated~, or all questions closed (not migrated)