A perennial issue for any online forum is moderation and applying rules. We have a set of community guidelines and posting guidelines that are intended to make clear what is and isn't acceptable but this tends to focus on the obvious standards of behavior for, well, obvious reasons. It's easy to talk about people being aggressive or insulting etc... as those are common and clearly unwanted behaviors and the typical issue for online communities - the most likely things to come up.
Does this mean that everything *not* on the list is allowed? That is how some people like to view it but it's unreasonable and typically demanded by people looking to cause issues, not get clarification on things.
Some things are removed because they are just noise or better served with a different format, e.g. a PM. An example is in fact "you have a PM" topics. It's completely unnecessary because they person will see if they have one. Does such a topic do any harm? Is it breaking any of the content guidelines? In itself, probably not. But the factor I always apply is whether things still work if 10, 100 or 1000 people start doing it. It would make the forum completely unusable which is why it's not allowed.
Another one is "Where is poster XYZ?". Occasionally it makes sense when someone returns after a long hiatus and wants to catch up on a well know poster from years ago but when someone hasn't posted for only a few days or weeks? It's over the top. They could be on vacation or taking a break - have you tried sending them a PM first and being patient? Chances are it's a question no one can answer - I'm the only one who knows if someone is visiting but not posting but I'm not going to tell you that. Again, if everyone started doing it then it would have a negative impact on the forum as a whole which is why it's not allowed.
Why aren't these in the rules? Because it's impossible to think of and list every possibility for every conceivable content and the more we tried to the more people would see the list as the definitive set of rules that then must allow anything not on it. If you're looking for loopholes then you have missed the point of the guidelines. The prime concern for me is keeping the forum usable and available for what it's intended - discussing topics and through that peer support.
So what if you make a post and it's removed? Unlucky - you found out it wasn't allowed! Why didn't I PM you and give you a thorough and detailed explanation? Because I couldn't be bothered to be honest. Seriously, where do I draw the line for who should get an explanation and who shouldn't? Am I supposed to explain every single action to every single person and then reply to every demand to explain "which specific rule did I break?!" ? Can you imagine how much work would be involved? This isn't my "job", we do our best, we rely on your cooperation. You are not the only thing happening, ever.
If you post something and it's removed, maybe stop and think "hmmn, could there be a reason that was removed?". If you are truly bothered and think a gross mistake has been made you could PM me and ask. What is probably the wrong thing to do is post it again ... and again ... and again ... at what count do you not start thinking "maybe I should stop" ...