You aren't going to get a nice first party source, although Apple has essentially admitted to it.
Apple says that "For all devices running iOS 8.0 and later versions, Apple will no longer be performing iOS data extractions as the data sought will be encrypted and Apple will not possess the encryption key."
You're going to interpret that as "oh hah, well some of the data wasn't encrypted before". But I included links earlier showing how Apple's (and Google's) announcement centered around the use of new technology that would not allow Apple to have a copy of the key. And if you look at the data they were pulling off, there was data that was 'encrypted' before but had engineered circumvention - 'active data' being one.
Apple will (currently) decrypt data it has in iCloud for a device. It is engineered to be very difficult to not store data in iCloud [1]. Apple's own list of information that may be in iCloud include "stored photos, documents, contacts, calendars, bookmarks and iOS device backups. iOS device backups may include photos and videos in the users’ camera roll, device settings, app data, iMessage, SMS, and MMS messages and voicemail."
https://news.ycombinator.com/item?id=8522316 for more.
Re: last time. I pointed to other areas of the thread, where plenty of sources were given.
[1] http://mjtsai.com/blog/2014/10/26/yosemite-uploads-unsaved-d...