I'm not too sure how this would be implemented. Icon sounds ok to me. Or color bar changing color over time--something that goes black if its not "bumped up". Or, a tag like was mentioned labeling items depreciated--maybe a staging for an archive mirror. Or a (five) point sliding scale of depreciation set by users.
I'm not advocating removal of content. I prefer to have access to archive content and judge myself if its useful for my purpose. Yet, I'm starting to imagine why forums like Apple.com (seem to) remove old content. It simply generates the same questions over again in a process of renewal.
I just remembered why this seemed like something I wanted to discuss--Drupal.org OMFG. That site make me go bat-$hit with the number of documents that are defacto out of date. Most searches turn up a host of outdated discussions, modules, issues mixed in with newer contributions. Its tedious.
Of course the base and reasonable practice for search result filtering is simply to leave it up to judgement with reading the results. Text description alone is low information density. Tags add another layer. Voting systems are another layer. Yet, as I suggest, I don't know of any web site who use voting systems for posts actually reedited to depreciate outdated content. Here is a funny example I found on marquee in css.
I can't speak for other users. Depreciation visual feedback would make a better search experience. Whatever it is I imagine should be visible in the top real estate. Which poses a strange issue where the last thread may be years late, and countermands the majority of replies. Yet, it is my experience resurrecting very old threads is frowned on.
I imagine as forums continue to be useful tools, more needs and ideas will be developed. My thoughts could also be premature for Sitepoint?