OK friends of outlining, here is a little report after upgrading to Yosemite, and Page 5.5/2.5 (iOS).
The good news is that Pages 4.2 (Pages '09) still works, as far as I can tell. I can open my old outlines and still work with them.
The bad news is Pages 5.5 still does not support Outline Mode.
Page 5.5 will still coexist on the Mac, and open 4.2 docs with a polite dialog that says outline mode is not supported.
One nice thing that happened a while ago was that iCloud would store 4.2 docs, and if you tried to open them with iOS Pages (the new one, which I unfortunately got stuck "upgrading" to), it would offer to make a new copy and let you read that in Pages iOS. Thus your original 4.2 doc on iCloud would not be corrupted to the new Pages. However, this functionality is lost in Yosemite, probably as Apple seems to be trying to maintain iCloud while bringing on iCloudDrive. iCloud docs are not longer retrievable in iOS, it seems. If you resave your 4.2 doc on the Mac to iCloudDrive, it is is not findable by iOSPages2.5. Apple pretty much forces you to upgrade to the new version of iOSPages when you "upgrade" to iCloudDrive, which you must do for the cloud features of Yosemite to work.
The wild thing is, all the templates of 5.5 are available in 2.5. In other words, you can do some pretty amazing layout things on an iPhone. As long as you don't want to outline, or read your outlined documents.
I still love outlining, and consider this lack, this killing, of outlining a giant step backward for Apple, the major thing that makes me want to give up on the whole environment. The fact that they offered it is what allowed me to get off Office, after 20 years.
To be clear, the minimum I want is to be able to create an outline on the Mac, and read the result when I am out and about. By outlining I do not mean something that looks like a type-written outline, but something where you can grab any part (header and following content) and move it around in the document. This includes being able to expand and contract text. It is a creativity tool, not a formatting tool.