Apple have responded to my issues with broken NSLocale property availability. The properties are actually convenience accessors which you can re-impement youself for use before iOS 10.

These properties are new in iOS 10, and you could add equivalent properties for older OSes using an extension like the one shown below:

extension NSLocale {
   var compatabilityCurrencySymbol: String? {
        return objectForKey(NSLocaleCurrencySymbol) as? String
   }
}

Although that might work, the argument isn’t sound: the properties were available before iOS 10. rdar://28363526