The iOS release of WhatsApp has implemented a new optional security feature that locks all use of the app behind either Touch ID or Face ID depending on which iOS device you're using the messaging app on.
The new version of WhatsApp Messenger, 2.19.20, describes how to enable this feature in the change log on its App Store page:
• You can now require Face ID or Touch ID to unlock WhatsApp. Tap "Settings" > "Account" > "Privacy" and enable Screen Lock.
This added security is a welcome feature for users that prefer the app to traditional SMS text messaging due to its usage of end-to-end encryption on all communication. Other end-to-end encrypted messaging services include iMessage, Signal, and Telegram. Of these apps, Signal also has Screen Lock functionality.