I'd suggest automatically marking old threads as closed. You could define "old" as "no followup messages in N months". One or two might be a good number. That would keep topics relevant and, oddly enough, more germane to the original thread. All too often I see interesting topics veer off course to the point where I stop reading them. When someone replies months later I don't know if they're responding to an off-topic reply or the original message.
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How To Roll A Blunt