Reminder: iMessage claims to be e2ee, but the on-by-default iCloud Backup on iOS backs up material that is sufficient to defeat this (either the endpoint keys, in the case of "Messages in iCloud" disabled, or the messages themselves, in the case of "Messages in iCloud" enabled).

This means that, in practice, iMessage is not e2ee.

Before you say "But what about Advanced Data Protection that enables e2ee for iCloud Backup?" - virtually nobody has this on, Apple prohibits you from turning it on in the UK, and even if you enable it - the people you iMessage with don't, so your conversations are in their backups. This means that if either endpoint of the iMessage conversation is in the UK, and both parties have iCloud Backup enabled (the default), then your iMessages are not e2ee as a non-endpoint has an escrowed copy of the plaintext or keys.