Messages in iCloud not implemented at present, coming in a future update to iOS 11
A feature touted for iOS 11 at the 2017 WWDC was Messages in iCloud -- but it was removed during the beta process, and will return at some undisclosed point in the future.

The "Messages in iCloud" feature synchronized a user's Messages "stack" between devices seamlessly. With the feature set implemented, deleting a message on one logged-in device would strike it from all, plus only the newest messages would be stored on-device with the rest held in iCloud and loaded only when needed.
At present, there is no mention of the cloud-based Messages on the feature page for iOS 11.
AppleInsider reported that feature was removed from iOS 11 during the fifth beta release. Apple claimed at the time that the purged feature will ship in a future software update to iOS 11.
Even without the functionality, users can continue to receive and store messages on each device, and can continue to backup and restore messages using iCloud Backup.
At present, not only is Messages in iCloud not fully implemented, but Apple Pay Cash and AirPlay 2 appear to be missing as well.

The "Messages in iCloud" feature synchronized a user's Messages "stack" between devices seamlessly. With the feature set implemented, deleting a message on one logged-in device would strike it from all, plus only the newest messages would be stored on-device with the rest held in iCloud and loaded only when needed.
At present, there is no mention of the cloud-based Messages on the feature page for iOS 11.
AppleInsider reported that feature was removed from iOS 11 during the fifth beta release. Apple claimed at the time that the purged feature will ship in a future software update to iOS 11.
Even without the functionality, users can continue to receive and store messages on each device, and can continue to backup and restore messages using iCloud Backup.
At present, not only is Messages in iCloud not fully implemented, but Apple Pay Cash and AirPlay 2 appear to be missing as well.
Comments
maybe they should just release new features throughout the year and forget the annual release cycle that they are stuck in.
In the iOS 11 version of iMessage, a side-scrolling list of apps appears appears below the text entry box for typing a new messaage.
On my phone, this list includes Google Maps but not Apple Maps. Both apps are installed on my iPhone, and I use Apple Maps frequently.
Strange... why would Apple's in-house Maps product not appear for pasting content into iMessage?
Did they fix all the text edit box errors (text, selection, navigation, autocorrect fails) in Safari (and now Mail, with iOS 10) in iOS 11??? How about the tab preview images? Or the printer offline dialog in AirPrint? Or the 3D Touch trackpad awkwardness on phones with 3D Touch (not just the pressure activation, it's another example of Safari's broken text edit fields). How about the autocomplete suggestions offering word replacement options when a word is user-selected but not actually doing anything when tapping one of the suggested words? Etc etc etc...
Just like Faces with Photos in iOS10....
Pulling something because it would be too buggy to release before they freeze features that will be in the .0 release is done all the time.
You prefer they release a POS piece of software?
In beta they determined that they could not predict fixing everything before 2-3 weeks before the first phones get shipped (they do need to put 11.0 on the phones in their boxes), so some things get pushed back. It is what sensible and reasonable companies do.
Release something mostly broken is not something you should ever do, but MS has done many times (though not in the last year).