Most forums out there discourage from replying to old threads , because it notifies all of the participants, who might have completely lost interest in it (and possibly forgotten the context, so they cannot make use of your reply anyway).
Another reason against necrobumping is, that systems evolve rapidly and therefore answering a two years+ old question (especially on Arch linux forum) is like telling Julius Caesar to use short-range subsonic cruise missile against Obelix (if you know the French comic).
Stack Exchange on the other hand explicitly allows it, because their Q&A often show up in search engines results and so people are likely to end up on the exact same old question. Because of that, they even advice to post relevant updates to old threads (like Hey, despite the accepted answer being the best one at the time of asking, you can now use this oneliner thanks to this RFC being accepted two weeks ago), because it is the simplest way to make the update reach those, who need it.
What is your opinion Nixers?