Facebook’s WhatsApp has introduced a new feature with version 2.19.20 sitting under Settings>Account>Privacy>Screen Lock that puts a “password” to your WhatsApp so nobody can open the app and read messages.
The feature is out for iOS devices as of now and rolling out to Android users very soon.
Depending on the iPhone version you have, when you open the app, it can either use FaceID (found in iPhone X and iPhone XS) or uses TouchID found in the previous iPhones such as iPhone 8 and prior.
So even if your phone has been unlocked, the WhatsApp remains protected until it is unlocked using FaceID or TouchID.
It’s a nice added layer of protection which I and many certainly appreciate.