I don't see any reason why replying an old thread should be against the rules. If the forum owner doesn't want old threads to be replied, he should lock them all.It depends on based on relevance. Some forums think it's against the rules, but I barely see why. Then again welcome threads of people who've never posted after I've been in at the wrong time. There needs to be a good reason for this stuff.
Why leave the thread open when they don't like necroposting? They are the one's enabling it the way I see it.Many communities typically don't like it when old threads are bumped (necroposting) regardless of whether they are answered or not, so I tend to play it safe and just let them be!
I wouldn't support the so called pruning because all it does is making the forum lose content especially if it was a thread with lots of engagements. Locking up the old thread will always be the best way to stop necroposting. Delegate staff members to lock up threads that haven't been replied to in 2, 3 or 4 years depending on how long the forum have been active.It's up to the staff on how they handle old topics! Staff can:
1. Lock old topics either manually, or automatically (either by forum extensions, or as a basic forum ACP (Administration Control Panel) feature if it's provided)
2. Set up certain forums to automatically delete all topics that have gone a certain time (specified by them) without a reply (which is a process called 'pruning') via the ACP
3. Restrict bumping via the ACP
4. Do absolutely nothing and just forbid necroposting as a general rule! Many communities often won't do the above since it can take time to set up such features, and it's easier to handle necroposts on a case by case basis!