It doesn't have to support old threads. From what I've seen discussion usually dies down after a few days, if that, probably because of a lack of such a system.

It doesn't have to support old threads. From what I've seen discussion usually dies down after a few days, if that, probably because of a lack of such a system. But even if it could be enabled for submissions no older than a week, it would be a great usability improvement over the current manual way of checking for new replies, or these external notification systems.