“For certain sensitive information, Apple uses end-to-end encryption.”
“These features and their data are transmitted and stored in iCloud using end-to-end encryption:”
* Home data
* Health data (requires iOS 12 or later)
* iCloud Keychain (includes all of your saved accounts and passwords)
* Payment information
* QuickType Keyboard learned vocabulary (requires iOS 11 or later)
* Screen Time
* Siri information
* Wi-Fi passwords
If “backup”, photos, messages, contacts, calendars, iCloud Drive, notes, and safari data (and a few more) are end-to-end encrypted what else is there?
The list of E2E is further down, separate from the table, and includes: Home data, Health data (requires iOS 12 or later), iCloud Keychain (includes all of your saved accounts and passwords), payment information, QuickType Keyboard learned vocabulary (requires iOS 11 or later), Screen Time, Siri information, and Wi-Fi passwords. So virtually nothing, by comparison.
Messages, probably the most personal and relevant for legal cases, are end-to-end-encrypted as well, but if you have iCloud Backup enabled, the key is stored in the backup, making this useless.
It would be nice if Apple was more forthcoming with that fact but there is some onus on the customer these days to understand what's private and what is not.
https://support.apple.com/en-us/HT202303
iCloud backups are not on the list of end-to-end encrypted.