Last year Meta introduced a new WhatsApp feature called Chat Lock, allowing threads to be moved from the Inbox to a separate folder, requiring the device’s passcode or biometric to access it. You can find out more at https://faq.whatsapp.com/764072925284841/.
Meta have now added a feature called Secret Code, where you can protect the chat with a passcode different from the device’s own, to add another layer of security.
We know that many of you use WhatsApp in places where security is an issue, so this might be worth a look.