At least 90% of internet forums out there have a strict rule against so-called necroposting. This is defined as responding to a thread that has not had any activity in a long time. (The amount of time varies from forum to forum, ranging from years to just a few months.) Necroposting is somehow considered a really bad breach of netiquette or something. If anybody necroposts, an angry swarm of people will immediately castigate the culprit with angry reminders that the original thread died several months ago! In fact, a few forums even go so far as to automatically lock threads that have not had any activity in a given amount of time.
I have never understood (nor will ever understand) what exactly is so bad about "necroposting". None of the arguments given against it make any sense.
So what if a thread has not been active in many months, or even years? Someone might still have something new to add to it. It could be a new perspective, a new idea or even an update of recent events related to the subject of the discussion. Posting it in the thread that discussed the subject keeps it in context, keeps the forum better organized and doesn't scatter discussions on the same subject on different random threads.
Someone might respond with something that has already been said in the thread (eg. because of only reading the first post and responding to it, instead of checking the rest of the thread first). However, this has nothing to do with necroposting. People do that all the time even in threads that have had abundant activity even during the last hour. Reprimanding such a person for doing that just because the thread happened to be old (but not doing so if the thread happens to be recent) is irrational, inconsistent and outright mean.
Fortunately there are some online forums that do not have any such nonsensical rule. Yet even in those you can see people complaining if someone necroposts (and then regular users telling them that "we don't have such rule here".) The irrational hatred of necroposting wants to spread even to forums where it doesn't exist...
There is also another, potentially positive aspect to "necroposting": It draws attention to old threads that might be of interest to new people. When a forum has thousands and thousands of threads, nobody is going to wade through all of them. If someone, however, makes a post to a years-old thread, it's often because the thread is interesting, and the post will bring it back to the top (or at least to the list of threads with unread posts.) Some people who had never seen that thread may find it interesting. (And this isn't just speculation. Recently in a forum I witnessed exactly this happen. Someone "necroposted" on a very old thread, and somebody else commented that the thread was actually interesting, and expressed gratitude on drawing attention to it.)
The aversion to necroposting needs to die. It makes no sense.