Any reason why HN source code is not published? The best I can think of is not to let people see the penalizing behavior, but having an open standard might actually help it improve rather than keeping it as a hidden secret that slowly gets discovered by independent malicious parties.
Good point. You can see it as a part of a "defense in depth" strategy. Ideally you want the system's security architecture to stand it's ground, even if everything (except for the private keys) gets leaked. In practice you'll conceal some of these details to gain an extra edge.
Lisp famously allows your software to be extremely dynamic, so I don't see why such functionality shouldn't be implemented as a closed plugin, while the core of the site would remain open to public contributions. And boy does this website need a couple fixes... (dark mode? mobile CSS? large comment thread performance? dupe link detection?)