It does go on to explain that threads in the introductions forum are considered "dead" after 7 days and shouldn't be posted in, the only other sub-forum it specifically targets is the Q&S forum where an exception is if you find a thread that's ancient and the problem you're having hasn't resolved with the answers in it.
You're not really going to see this type of rule break in other parts of the forum because of just the way the forum works. Most everything else is made up of things like forum games, general discussion threads, journals, actual role plays - things we're not going to put a hard time limit on because they understandably vary with traffic and can be picked up/dropped at any time.
Visibility of specific threads/information is going to be one of the most important reasons. The two forums specifically listed in the rule are important to keep clear of clutter and "dead" threads. Someone who is brand new to the site could make an introduction thread and then someone goes on a necroing spree and buries the brand new intro thread onto page 2 before someone from the Welcoming Committee can make sure they get shown around. Similarly, the issues/questions/suggestions that are on the front page are all current and we don't want them to get buried by something already handled.
For the most part, it's not a rule that we see get a lot of breaks and most of the time is just a harmless mistake that we correct.
Adding a banner to all old threads or a pop up is more software limited by xenforo and just isn't really feasible or necessary. It takes a handful of seconds to see the time stamp on the last post and make a judgment decision on whether this is something you should be contributing to or not.