I submitted an iMessage sticker extension to Apple for review. They rejected the submission with the following explanation:
Specifically, we continue to find that one or more stickers in the sticker pack contains potentially objectionable content.
Stickers should not offend users, create a negative experience, or violate the law.
I replied back and asked if they could tell me which designs specifically were offensive, however, they stated that they will not provide this information.
My question is where can I find some sort of guidelines about what is and is not permissible? The human interface guidelines provided by Apple for messaging extensions does not seem to outline this information.