It’s called “advanced data protection”. You’ll find it hidden deep within the iCloud settings.

It’s off by default.

Wow, thanks for mentioning this. I was also under the impression it was e2ee by default but it was actually disabled for me.

Interesting. FYI, I'm not an apple user, but no upset that some are.

I wonder. Is this an attempt to market? Make it optional, but tout "we do this!', as if it is?

It's not a bad marketing position.

Actually there is a third option, don't back-up your iMessage to iCloud in the first place. In this configuration you need to transfer your content from device to device using a local backup if you intend to keep your messages.

You the have the same level of privacy (if not higher) than with ADP. But with the same drawback, if your recipient does backup to iCloud without ADP then messages can be intercept by apple at rest on your recipient iCloud backup.

Incidentally ADP mainly target users that didn't trusted iCloud backup for the lack of e2e encryption at rest.

More prosaically it's probably because turning it on means it's easier to lose access to your data if you lose devices/keys. Apple can't help you if they don't have the keys. It's not a bad marketing position.