By default, WhatsApp chats are end-to-end encrypted, but the backups aren't encrypted for iPhone users. To be on the safe side, you can enable end-to-end encryption for those WhatsApp backups. Here's how.
Start by opening WhatsApp on your iPhone. Next, tap the "Settings" (gear-shaped) icon in the bottom-right corner.
In WhatsApp Settings, select "Chats."
In Chats, tap "Chat Backup."
Under Chat Backup options, tap "End-to-End Encrypted Backup."
The next screen will show that the Encrypted backup is off. Select "Turn On."
On the next screen, tap "Create Password."
Type a new password with at least 6 characters and one letter. Then, tap "Next" in the top-right corner. Retype the same password on the following screen, and tap "Next" in the top-right corner again.
Make sure you write this password down somewhere safe. If you lose it, you will lose access to your message backups.
Tap the "Create" button at the bottom to confirm that you want to create a new End-to-End Encrypted Backup. WhatsApp will prepare your backup with encryption and store it in iCloud.
That's it! When you want to restore your WhatsApp messages backup, you'll have to enter the same password that you've set for your encrypted backups. Good luck!