The latest update to the WhatsApp for Android beta app has finally enabled biometric authentication. You can enable the new feature by navigating to Settings > Account > Privacy > Fingerprint Lock in version 2.19.221 of the beta app.
Once there, you'll have the option to unlock WhatsApp using your fingerprints, almost seven months after WhatsApp enabled the same for iOS users vis-à-vis Touch ID and Face ID. Alongside an on/off toggle, you can also set the frequency with which the app will automatically be locked and require you to unlock it again.
For those concerned about privacy, there's also an option for WhatsApp to not show any content in the notifications pane or lock screen notifications while the app is locked. The same is true of any widgets you might have placed on your home screen.
Unfortunately, it seems that not all WhatsApp beta users are seeing the option, even with the latest update installed. So, if you're on the beta for the app and can't wait to try it out, be advised that your luck might vary. WhatsApp will hopefully make the feature available to all users sooner rather than later once testing is complete.