According to a report from 9to5Mac, they have learnt that the “inappropriate content” in question was child pornography. This is according to an email that one of their readers sent to Apple executive Phil Schiller (which has been verified), in which Schiller explained that the removal was due to child pornography being distributed by certain users of Telegram’s app.
Schiller’s email reads, “The Telegram apps were taken down off the App Store because the App Store team was alerted to illegal content, specifically child pornography, in the apps. After verifying the existence of the illegal content the team took the apps down from the store, alerted the developer, and notified the proper authorities, including the NCMEC (National Center for Missing and Exploited Children).”
He also adds, “We will never allow illegal content to be distributed by apps in the App Store and we will take swift action whenever we learn of such activity. Most of all, we have zero tolerance for any activity that puts children at risk – child pornography is at the top of the list of what must never occur. It is evil, illegal, and immoral.”
That being said, Telegram is back on the iTunes App Store following their removal of the illegal content, the ban of the users who were sharing said content, and also putting into place more controls to prevent such activity from happening again.