E2EE is optional, and disabled by default. Out of the box, your iPhone will store your messages in cleartext on Apple servers every time it backs up.

This is a twisted way of seeing it.

E2EE is mandatory in Messages.

Backup is optional - my phone has never seen an iCloud backup (all local backups).

No, it's not. By default, iMessage messages are stored in cleartext on Apple servers. You need to use an option, to disable iCloud backups, so that this isn't th case. So the messages are only E2EE if you take care to change the options so that they are, otherwise they are not.

I don't think I'm twisting it. Pragmatically, unless you fiddle with the options, iMessage won't give you end to end encryption. So I think it's fair to call it optional.