tl;dr: this rule already exists, though in a way that's slightly but importantly different: if the post adds something new and worthwhile, allow it and if it doesn't, don't.
While I could just copy and paste the rule that is already listed on the site and leave it at that for an answer, I wanted to write something that was more thought out and gives an insight into why this rule actually exists in the form it does. In my opinion, this rule is rather misunderstood, and I feel that some good content has ended up being flagged or voted down as a result of this. I want to try and differentiate between posts on old questions that are accepted, and the ones that are unhelpful and that you guys should flag for us to notice and hide. I might not speak for Pokemaster here, but I still want to put this forward as I think that we should be more accepting of some of these responses to old questions.
I firstly want to say that in my opinion, a rule like the one you've brought up might not work very well. If that's how we'd have liked old questions to be treated, those threads would be closed -- I know the software used here is capable of that, but it isn't currently implemented or used for this purpose. The reason for that is that we allow, or even encourage, users to add to old posts if they can correct or improve upon an answer on contribute something of worth in the comments. Adding a blanket rule like "don't post on old threads" basically bans these posts, and would go against the notion that these posts are made accessible to anyone ever and should be able to provide the best answer possible regardless of when it was posted. Obviously we have to make reservation for posts that could constantly be updated or have varying answers for different games (eg "where is this item located?") as these would constantly be bumped up, but otherwise, people can certainly go ahead and add answer to old posts with bad answers.
I've personally approved several new answers to posts from as early as 2010, because the answers back then were admittedly of a far lower quality than we generally accept nowadays. This is one of the reasons why I think people should be able to make posts on those threads -- as stated before, those posts are available forever and serve as the site's permanent response to those queries. Rules like "don't post repeat questions" basically depend on there already being a good quality answer to the question on the site, and if there isn't, what's the point of that rule anyway?
The rule that I think fits best and balances the ability to make posts on old questions and the requirement to keep new posts on the top of activity lists is already included in the rules as follows:
Avoid duplicate answers: If there is already an answer on a question there is no need to post unless the current answers are wrong or you can significantly improve on them. Just upvote the best answer(s). (Note if multiple people end up posting answers in a short space of time it doesn't matter.)
This rule fits perfectly in my opinion, firstly because it does not discriminate between posts for their age. Realistically, how is different is a post that was resolved five minutes ago to one that was resolved six years ago? The only practical response to that is that the asker is more likely to see the answer, but remember that the asker is not the only person who could benefit from the answer. For this reason, the rule we use should not be, "don't post on old questions", but instead "don't post on resolved questions" like the rule above suggests. Secondly, this rule leaves the door open for users to add answers if they think existing ones aren't accurate or are lacking detail. Again, these posts should always have the best possible answers available on them, so if people are adding posts that are worth bumping the thread for, new responses should absolutely be accepted.
I guess since I've basically just green-lit necroposts as a whole, I guess I should explain which posts are the ones I wouldn't accept, huh? I generally go by the following criteria for this: if the existing answer/s is incorrect, and the new answer is correct, the new answer should be accepted. If it's incorrect, it should be flagged and hidden. If they're both correct, but the new answer adds new detail that makes the existing answer noticeably less helpful by comparison, it should be accepted. If the answer is correct but mostly a rehash of an existing post or doesn't add substantial detail, it's not worth bumping the thread and should be flagged and hidden.
Comments on old posts generally follow the same rules, but perhaps could do with a separate mention in the rules since they do bypass moderation and are probably the most disrupting to the site's navigation as a whole, since bad new answers generally come from new users and will be rejected in the admin. Basically, if the comment isn't contributing anything new and important, it probably shouldn't be accepted. I see a lot of people asking further questions on old posts through the comments; those people should probably use the big "ask a question" button instead of bumping an old thread. If the question is the user's own or if it's fairly recent, it makes more sense to post in on the answer itself since a response from the same user is more likely.
Also, if people have not read the rules and didn't know that unhelpful new posts that don't contribute anything weren't accepted, that's their problem not ours. And sorry that this ended up so long ;P
