I haven't been able to find a definite answer to this question.
I've upgraded my iOS devices to v9.0 and in both of them iOS "Documents and Data" is nowhere to be found. I got the prompt to upgrade to iCloud Drive, which I didn't do (*). When I installed iOS 8, I also didn't update and things worked as they did before.
Now I seem to be stuck in a weird limbo as Documents don't sync via iCloud but I haven't been able to find any definite answer on whether this is by design (I find it quite surprising as it's a bit of nowhere land). Or a weird iOS 9 bug.
Also, on the Mac/OSX front, is there any confirmation on whether Yosemite/El Capitán onlysupport iCloud Drive too?
(*): I have a work Mac running Mavericks that I can't upgrade to Yosemite, and a Mac Mini on Snow Leopard that I have the feeling won't be able to run Yosemite with anything resembling reasonable performance.
Best Answer
Short answer is that your intuition is correct: in iOS 9 and Yosemite (or later), you have the choice between iCloud Drive or no iCloud at all.
The reason for this is that "Documents & Data" syncing was re-written using the new CloudKit in iOS 8 and OS X 10.10, and renamed "iCloud Drive".
iOS 8 did remain backwards compatible with old-iCloud if you chose not to upgrade to iCloud Drive. One weird exception to this was the iPhone 6 and 6+. These devices did not have this compatibility mode, so it was iCloud Drive or nothing.
OS X Yosemite (10.10) also does not have the same compatibility mode though, so unless iCloud Drive was enabled, there is no document syncing. The same goes for OS X El Capitan (10.11).
I expect that the iOS 8 compatibility was only included to fill the gap between the time iOS 8 and OS X Yosemite shipped, so I cannot imagine that iOS 9 would be compatible with the old-iCloud in any way.
Also note that "iCloud" is a collection of different technologies under the surface, so things like Contacts and Calendar may be able to sync without iCloud Drive enabled on all devices, but most other apps will need it enabled.