WhatsApp has now come up with yet another update. As per the update, WhatsApp can now be protected by using your phone's biometric security. You will now get an option to either use Face ID or Touch ID through which you can restrict access to the software.
This feature is available with WhatsApp version 2.19.20 and can be enabled from the app's Privacy menu within Account settings. The new layer of biometric security should give the contents of your messages more protection even if your phone is unlocked.
As of now the feature is available only within the iOS version of the app. However, an Android release is soon to be expected.