The bug came to light when security researcher Patrick Wardle received a message from a Taiwanese friend, reporting that iMessage, WhatsApp and Facebook Messenger all crashed when she typed the word 'Taiwan' or received a message containing the emoji for the Taiwanese flag.
On an iOS device with CN (China) set as the language/locale, iOS is looking for the Taiwanese flag emoji and then removing it. That code was buggy, which was what caused the crash. [...]
The company has been accused of putting sales ahead of human rights, agreeing to a long-running series of compromises to satisfy the Chinese government. The most controversial of these was moving the iCloud data of Chinese customers to a server run by a state-owned company, reportedly also handing over the encryption keys. Apple has also removed or restricted apps in the country -- including more than 400 VPN apps.
iOS bug reveals how Apple censors the Taiwanese flag on Chinese iPhones: