Earlier quoted context omitted.
iCloud Backup backs up plaintext of all iMessages from the device to Apple, with Apple keys (non-e2e). It also includes all SMS. This is documented plainly on Apple's iCloud security overview page. The list of things that are end-to-end encrypted is listed. iCloud Backup is not in it. It's on by default on all iOS devices. Apple was to fix this by end to end encrypting iCloud device backups, and (I understand) even h…
iCloud Backup being insecure does NOT mean iMessage is "backdoored": if you don't use iCloud Backup--and there is no reason to do so, as Apple (notably unlike anything Android) has a really good local way to do highly-secure backups using iTunes--my (rather firm) understanding is that, even if you turn on the iCloud iMessage sync (which is also optional: I do not have it on, for example), iMessage actually is pretty…
Technically not, but in practice it is for the most users.