[sneak]

iCloud Backup backs up the entirety of someone's iMessage history (or their endpoint iMessage sync keys, which equate to the same thing) each night to Apple without e2ee.

If you enable e2ee for iCloud/iCloud Backup, all of your iMessage traffic will still be escrowed, simply from the other end of all of your conversations because they still have iCloud e2ee turned off (because it's off by default).

If the endpoint sends the plaintext post-decryption to the middle transit service (Apple) in a way that is readable to that middle service (iCloud Backups contain complete iMessage history and are encrypted to Apple keys), then it's not e2ee. This is called "plaintext escrow".

Same goes for WhatsApp. It backs up its message history to iCloud or Google Drive, which are, in the usual case (99.9%+ of users) non-e2ee.