This is still an ongoing issue and Apples silence on the matter is deafening. I know they don't respond on these forums but this is a gaping hole in iOS settings and needs fixing!
I have a rudimentary fix for this however but will only be useful to a handful of people in specific situation. That being if you have given your kids a phone and don't want them to access data for fear of uncapped tariffs racking up huge bills.
Go into the mobile data setting > mobile data network and then erase the APN address under the mobile data section at the top. Make a note of it so you can re enter if need be later on.
This will however stop ALL mobile data as there is no server address for your phone to access. Another issue is a carrier settings update may get pushed to the phone and update the APN itself so you will still have to keep an eye on it a bit but it will help to stop mobile data being used for the most part.
I do this on both my kids phones as iOS ***** big time as we all know and my crappy carrier refuses to put a cap in place.