The new setting can be accessed by updating WhatsApp to the latest version, then launching that app and tapping the Settings tab. Talking more about the feature, it is present in a new "Screen Lock" section in the "Privacy" section in "Accounts Section". The app leverages the Face ID or Touch ID feature on your phone to lock the app. In the future, Facebook plans to create a unified messaging system that would work across Messenger, WhatsApp, and Instagram, complete with end-to-end encryption. The WhatsApp lock has been already rolled out for the iOS app and iPhone and iPad users can use Face ID or Touch ID to lock their WhatsApp.
Notably, Mashable's own Ray Wong recently got the ability to use Face ID with his iOS Instagram app; Instagram confirmed to us they're testing the feature.
There's no password backup option, which means that only your fingerprint or face will unlock the app. The feature is yet to roll out for Android users. The folks over at WhatsApp have chose to add an extra layer of security for those using the app and allow them to lock the app behind Face ID or Touch ID. Messages will still be partially visible via notifications, and they can still be quick-replied to. You can also select the lock timing accordingly. Getting into the details about the feature, iOS users need to be running version 2.19.20 for iOS.