We were alerted by Apple that inappropriate content was made available to our users and both apps were taken off the App Store. Once we have protections in place we expect the apps to be back on the App Store. — Pavel Durov (@durov) February 1, 2018

Telegram has had an extremist problem for quite some time. The company is constantly shutting down ISIS channels and it keeps butting heads with the Russian government, which has claimed that terrorists use the app to plan attacks. It has also had to respond to violent content, suspending an Iranian channel last year that was encouraging its subscribers to engage in violent protests. Because Telegram offers encrypted messaging as well as public channels, private messaging and secret chats, it's ideal for anyone who wants to protect their communications, including extremist groups. While we don't know if that sort of activity is what got Telegram booted from the App Store, it's an ongoing issue that the platform has had to deal with.

We've reached out to Apple and Telegram for more information. We'll update this post when we hear more.

Update: Pavel Durov shared on Twitter that Telegram is back in the App Store. Telegram X has returned as well.