Over on Gamefaqs, when a thread in any forum category has been dead for at least a week or two, it immediately gets archived and cannot be interacted with at all. You cannot respond to it, you cannot like it, you cannot click on member details, it's just -blam!-ing dead. All you can do is stare at it and realize you cannot do anything to re-bump it. We really should have had a feature like this by now, so people can't bump threads from 2009 if they feel like it, seriously.
Why hasn't this been done by now?
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I think that topics are now left open due to the notification feature. It means that old topics, ones that we used to consider "dead" no longer need to be locked or killed. If a member is away, comes back, sees a notification and wants to reply to a post someone directed at them months ago, they can, and the person to whom they are replying will then get a notification that their conversation is "back in business".