Comment by lloeki

CKV aims to cover that case:

https://support.apple.com/en-us/118246

https://support.apple.com/en-us/118247

> An unrecognized new device was added to that person’s Apple Account. This alert might mean that the person you are messaging has an issue with one of their devices, or that a sophisticated attacker might be attempting to eavesdrop on the conversation.

My understanding is that iMessage implements PFS. To get around PFS and access older messages, one needs to get their hand on a backup, which needs fully enrolling a device, not just messaging key exchange hackery.

And as far as trusting Apple with key exchange, well, if you're running their OS and hardware, I suppose that trust of key exchange is the least of your concern (or part of the whole deal anyway depending on how you look at it)