it did add Safari private browsing mode :), but overall iPhone piggybacked off of the Internet, which was created by Military and adult industry were the first large scale content websites that drove innovation
There are nevertheless viewers of such contents on iOS. They pass reviews so long the reviewer testing account only shows SFW sections that nobody uses. The users navigate from such sites to the App Store, and never the other way around, so there's absolutely no need for the app to have anything exposed to App Store.