We were talking about making a missile (v2) with an extended range, and ensuring that the developers who work on it understand the assumption of the prior model: that it doesn't use free because it's expected to blow up before that would become an issue (a perfectly valid approach, I might add). And to ensure that this assumption still holds in the v2 extended range model. The analogy to Ariane 5 is very apt.
Now, there can be tens of thousands of similar considerations to document. And keeping up that documentation with the actual state of the world is a full time job in itself.
You can argue all you want that folks "should" do this or that, but all I've seen in my entire career is that documentation is almost universally: out of date, and not worth relying on because it's actively steering you in the wrong direction. And I actually disagree (as someone with some gray in my beard) with your premise that this is part of "rigorous engineering" as is practiced today. I wish it was, but the reality is you have to read the code, read it again, see what it does on your desk, see what it does in the wild, and still not trust it.
We "should" be nice to each other, I "should" make more money, and it "should" be sunny more often. And we "should" have well written, accurate and reliable docs, but I'm too old to be waiting around for that day to come, especially in the age of zero attention and AI generated shite.