For a few years, WhatsApp "e2ee" messages were stored in plain text on your Google drive backup (that's how it worked on Android. Don't know about apple). This was even stated in their FAQ.

This was as part of a FB/GOOG deal where the storage for WhatsApp backups did not count for your Google drive quota.

Recently the backups did finally become encrypted as well. With a key known to the WhatsApp app. (On Android, stored in a file called "key" in the apps local storage)

However, when you restore the backup, where does the key come from? From the WhatsApp servers, obviously.

So still, FB and GOOG together still have full access to your daily backed up messages.

And the free storage deal is still there, of course.

Please do correct me if I'm wrong and you know better.