500px has returned to the App Store after its short hiatus over nudity concerns. Apple's main issue was that nudity was too easy to come across without any warning even though a recent update has made them harder to find, not easier. This time, 500px has been updated with an added mature rating and better photo reporting features.
It isn't blatantly apparent what changes have exactly been made to the 500px app other than it's had a mature rating added and an easier way to report photos. This was done by a "report photo" button being added in an easy to find place. This could be to address the child pornography concerns that people were rumoring. 500px stated previously that they'd never received such complaints but would take any very seriously.
Whatever 500px did to prevent nudity coming up unless a user wants to see it must have appeased Apple enough to allow the app back into the App Store.