You should see "WhatsApp Messages" under "Documents & Data".
Then go to iPhone Settings > Apple ID > iCloud > Manage Storage.
Ensure you've taken a backup of WhatsApp on iCloud in advance (Go to iPhone Settings > Chats > Chat Backup > Back Up Now).
How to Check iCloud WhatsApp backup on your iPhone: Then you could check iCloud WhatsApp backup on your iPhone or your Mac as follows. Where is WhatsApp backup in iCloud? To find it, you should first go to your iPhone Settings > switch on "iCloud for WhatsApp" > your name > iCloud > APPS USING Storage. Part 1: How to Access WhatsApp Backup on iCloud
Extra Tip: The Best Way to Back Up WhatsApp Data.
Part 4: How to Access Local WhatsApp Backup.
Part 3: How to Check WhatsApp Backup on iTunes.
Part 2: How to Find WhatsApp Messages in Google Drive.