If you have an iMessage chat that includes any Chinese user, you should consider it compromised. Whoever controls the key server with the users' public keys can eavesdrop on the conversations. For Chinese users, that is a Chinese company controlled by the Chinese government. For non-Chinese users, that is Apple. https://threatpost.com/apple-imessage-open-to-man-in-the-mid...

As far as whether your iCloud data becomes readable by the Chinese government if you travel to China, that is a reasonable fear and one that Apple should address. It is possible, even likely, that the great firewall blocks iCloud servers outside of China, which would mean the only way to access your data would be to have it available on servers that the Chinese government can also access. https://www.forbes.com/sites/emmawoollacott/2018/02/24/chine...