"Case law" is just a metaphor. The official rules are at
https://news.ycombinator.com/newsguidelines.html (that would be "the law", in the metaphor), but they leave a ton of questions unanswered—there are many complexities and nuances, too many to list, and they only really make sense if you talk about them in the context of specific examples (those would be the "cases", in the metaphor).
Since people ask about specific examples all the time, and we always want to satisfy their curiosity, I post replies that go into detail about how we think about moderation, how what we did in any specific case relates to the guidelines, and ultimately how it all derives from the single thing we're trying to optimize HN for, which is curiosity (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...).
Over the years, those replies have grown into a body of explanations that add context to the site guidelines and the other brief things that have been 'officially' published about HN. That is analogous to how case law (the specific examples of how laws have been applied in the past) adds context to legal codes, which as you say can be inscrutable—they need examples to make sense. Another metaphor one could use for this is hermeneutics or midrash, but that has religious associations which would lead to distracting objections, so I don't go there. Yours is the first objection I remember anyone making to "case law"!
Of course this is not formal documentation, but it does contain all the explanation anyone could ask for—detailed answers to every conceivable question about HN moderation; just not in an easily discoverable form, as you say. That's why I'd like to compile this material into a more accessible format. We'd probably do that instead of making a public moderation log of every mod action—to come back to your original question—because it is more likely to help people understand what they're seeing. I've been waiting for the answers to converge into something that's worked-out enough to deserve publishing, but that has started to happen.
No one is expected to read that stuff, let alone find it for themselves; but I do include links to past explanations in current answers, so that anyone who wants to read more can click and get to them fairly easily. For example, here's such a link regarding the point I made in the previous paragraph: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que.... You'll notice that it contains the current comment, as well as 3 past ones on the same issue. It's an informal mechanism and it doesn't work perfectly (because the search links can also dig up extraneous stuff), but it's a lot better than nothing and has proven to be a good way to spread knowledge amongst the community—which is a hard problem btw.
I get why you might feel offended if we were telling you "just go dig up your answers in HN search"—that would be a little like customer support telling a user "look it up in the code, it's on Github". What we're actually telling you (and all users) is: if you have a question about how HN moderation works, just ask. If I see your question in the thread, I'll be happy to answer it—often at length, as I've done here—but we don't see everything in the threads, so it's better to email hn@ycombinator.com. The answer might end up including some links to past explanations, but you don't have to dig them up—we do that for you.
Although this mechanism is messy and insufficient, it has an interesting advantage: knowing that explanations can be reused in the future allows me to answer specific users' questions in much greater depth. If the only people reading this were you and the few others who ended up in this obscure corner of a thread while it was live, it wouldn't make sense to spend an hour writing an essay-length answer. But because the answer is helping to build a corpus of reusable explanations, the "economics" work: it's an investment in future readers in addition to current readers. Sometimes I take this to extremes, as with https://news.ycombinator.com/item?id=27162386 from a couple weeks ago—that was a lot of writing for answering a single user, even though we value single users. But it was also a big step in expanding the "corpus", making it worth doing.
It is a nice feedback loop: individual users benefit by getting richer explanations, the "case law" (can I use that term now?) benefits by getting a new detailed entry (a worked example, you could say), and the previous examples can be linked back to, making future explanations more meaningful.
This "system" emerged spontaneously over many years, in a bottom-up way very much in keeping with the exploratory, hackerish spirit that animates HN (at its best). That's what makes it so weird and esoteric, but also why it's alive and it works. Indeed, it's the only reason why any rich body of HN explanations exists at all. A top-down, bureaucratic approach would have led to "policies"—more the line of the manichean archenemy of the HN spirit. And anyway we'd never do that in the first place.
This approach has even changed how we moderate HN: it has evolved into a continuous, multi-sided dialogue (multilogue?) between the moderation subsystem and the community subsystem, that goes deeply into the why of things, tries to discover underlying principles and reflect them back to the community. For example, it led to "we're trying to optimize for just one thing", which I linked to above. This dialogue shapes the community in turn—it helps the forum regulate itself, even (I believe) when moderators aren't present.
The next step is to mine this material out of the subterranean thread-niches it's currently buried in, and to "scale" the economics by compiling it into more definitive forms that can be linked to and browsed. Perhaps it will look like an extended HN moderation FAQ or blog. That will be easier for new users to find and hopefully also save us a lot of time in the future, because as I said above, the answers have started to converge, which makes them more repetitive.