iMessage's end to end cryptography has been backdoored via iCloud Backup, on by default since 2011. It uploads complete message history to Apple (even SMS, which they would not normally see) with Apple keys. Even if you have it turned off, your conversation partners won't.
> To access your data on a new device, you might have to enter the passcode for an existing or former device.
> Messages in iCloud also uses end-to-end encryption. If you have iCloud Backup turned on, your backup includes a copy of the key protecting your Messages. This ensures you can recover your Messages if you lose access to iCloud Keychain and your trusted devices. When you turn off iCloud Backup, a new key is generated on your device to protect future messages and isn't stored by Apple.
I didn't realize messages was an exception to storing the key on local devices.
Have your Apple friends install Signal, too.