Sure, I do contracting work for the military. There are hundreds of millions of secrets kept every day with hundreds of thousands of people keeping their mouths shut. Leaking is exceedingly rare.

D-Day and the operations around it? Various price fixing cartels that were eventually prosecuted?

Naming conspiracies that have never leaked is kind of hard, since we wouldn't know about them. So you have to look for examples of things that were talked about after they stopped being relevant, or that were uncovered without anyone blabbing.

enron maydolf haliburton its literally everywhere you look

JFK, 9/11, and Epstein spring to mind for major conspiracies. I think anyone with a head on can see the government bodies tasked to investigate those affairs were rife with conflicted interests, duplicitous individuals, and some intent that they should be as narrow investigations as possible. Those secrets have been kept or at the very least the limited hangout worked so well that people think only nuts question them.

I’m not sure I understand your question. Can you state it in a different way?

Would it actually be illegal to cover up/lie about? I assume the company is US based, there is certainly breach regulations in Aus (with a 12 month notification window). I guess if it’s publicly traded then it would be a breach of law, but what about if it was privately owned?