Communication between people with Apple devices using Apple’s iMessage (blue bubbles in the Messages app), are end-to-end encrypted, but the backups of those conversations are not end-to-end encrypted by default. This is a loophole we’ve routinely demanded Apple close.
Yikes. I didn't know that.
The good news is that with the release of the Advanced Data Protection feature, you can optionally turn on end-to-end encryption for almost everything stored in iCloud, including those backups (unless you’re in the U.K., where Apple is currently arguing with the government over demands to access data in the cloud, and has pulled the feature for U.K. users).