The latest WhatsApp update for iOS has added the ability to lock the app using Face ID or Touch ID. Since iOS itself locks a native app locking ability, it is up to developers to add this feature to their apps.
If you own an iPhone X or greater, you can use Face ID to unlock WhatsApp while iPhone 8 and lower devices will make use of Touch ID. Similar to LastPass, 1Password, and other apps, once you enable Screen Lock on WhatsApp, you will have to confirm your identity using Face ID/Touch ID every time you open WhatsApp.
You can enable Screen Lock in WhatsApp on iPhone by going to Settings -> Account -> Privacy. Do note that even after enabling Screen Lock, you will be able to reply to WhatsApp notifications from the Notification Center.
Make sure to update to v2.19.20 of WhatsApp from the App Store to gain access to the Screen Lock feature.
WhatsApp is among the most popular messaging service in the world. While it is not widely used in the United States, it is the messaging app to use in Asian countries like India and neighboring regions. The Facebook-owned messaging platform processes over billions of messages on a daily basis.
Download: WhatsApp for iOS