> if you broadcast clearly that undocumented behaviors are subject to change, then users can decide if they want to accept that risk

That sounds nice in theory, but doesn't really work in practice. If you're building infra and a core piece of your company's product relies on these undocumented behaviors, you can't just change the behavior and shrug your shoulders when the whole product breaks. Similar if you're providing an external API to users/customers, you can't just break their stuff without worrying about it.