No, there’s no obligation to memorialize everything in writing. But it’s a two way street. If a conversation is important enough, the parties also have incentives to write it down (to avoid recanting or conveniently forgetting or misunderstanding important terms).

In court, testimonial evidence is usually presented under oath, under penalty of perjury. If a company is subverting recording information by only discussing sensitive items in person, a good lawyer will put multiple witnesses on the stand, ask them all the same questions about the sensitive topic, then poke holes in the inconsistencies and see who cracks first on the witness stand. It might be hard to get someone to disclose the sensitive subject, but it’s rather easy to catch someone in a lie or a cover up. That’s usually good enough.