This would be just like how edit perms are currently disabled after 6 months already, and would prevent blatantly bad necroposting. Typically a post is no longer relevant after this timeframe, so it would make sense to disable commenting (especially since it’s already enforced for editing).
PMs as well as #lounge threads can be exempt from this, as well as anyone with a Leader role being able to re-open threads after this duration of course.
Side suggestion: a warning should pop up when you’re replying to something that is at least a week old. All you have to do is acknowledge it to proceed of course, but it would stop a lot of accidental necroposts as well.
Warning: the thread/comment you are replying to is _____ old. Are you sure you want to send your reply?