Ha, in Europe WhatsApp is even used by doctors and teachers to discuss patient/student matters, and even some government offices use it. It's wild how entrenched it is in every aspects of society, from social to business. Goes to show you how far good UX, simplicity and ease of use can take you.
But it looks like this lawsuit is exactly about the opposite, that messages cannot be accessed and reviewed easily. It's also easy to understand why banks prefer using secured applications like Signal when discussing secret deals rather than taking the risk that such conversations leak to e.g. competitors...