For reference I've googled every combination of "nsa apple mobile OS leak" I could think of and couldn't find a primary source.
http://www.spiegel.de/international/world/how-the-nsa-spies-...
Helps to type in just what you want and what will specifically have your answer. Mobile will give you garbage most of the time. Apple as well. A technical document will usually reference iOS. Also, you can use quotes to ensure something appears.
Interesting enough, me typing what you typed into Google still led to same leak and others showing potential backdoors. Hmmm.
"About the linked article: out of date (2013, mentions iOS 4.3.3). Very thin on actual information. "
It's what I got out of a quick Google. I was unwilling to spend more time on that angle as my list of risks plus Apple's development practices shows we should consider it untrustworthy by default. I just don't feel like putting too much time into finding the specific evidence NSA might hit a specific version of a product that wasn't secure in its entire history. Also, which came from a company whose products did things like require an admin login on certain services but not check if password matches records: just the existence of a password in submission was enough. Better to spend that time on researching actual security. ;)
Far as parallel construction, let me see if I can quickly Google something. Here's you a few on it early on with FBI and DEA's stronger cooperation.
https://www.techdirt.com/articles/20150427/11042430811/nsas-...
http://www.reuters.com/article/us-dea-sod-idUSBRE97409R20130...
Further, they'll actually let a criminal go just to prevent either the public or courts learning the details of defense-related techniques. The Stingrays are a perfect example:
http://arstechnica.com/tech-policy/2015/04/fbi-would-rather-...
However, your original comment made it sound like we had direct confirmation of that fact. I hadn't heard that before, which is why I was interested.