@Tes yeux noirs,
I don't think this is a matter of standard vs. informal English. I think it's a structural issue. I do understand what you're saying '(all) about' being used in a question about the reason for something unexpected. The sentence in discussion doesn't have the same structure as 'what was that about?'.
What is /this letter/ all about?
What is /this request for payment/ all about?
What is /the payment this letter is requesting/ all about?
etc.
I get it.
But
Quote:What is this letter requesting its payment about?
looks like a combination of multiple sentences, almost like in 'before & after' fashion:
1. what is this letter requesting?
2. what is this letter requesting its payment for (instead of 'about')?
3. what is this letter about?
...
You know what I mean? If you define informal speech as any types of mis-spoken sentences that are understandable due to contexts, pretty much all mistakes become informal speech. I'm afraid your definition is too broad.
I've asked quite a few questions by now. Most people agreed that many of my examples are not good English - they're either grammar errors or just too weird and awkward for use. You're the only one who said they are okay in either formal or informal British English. Are you British?