How do I enable/disable the handwriting feature in iOS 10 or later?
Apple introduced the ability to send handwritten text messages through the Messages app in iOS 10. While this is an interesting feature which allows users to personalize their text messages and replace handwritten physical notes, it can sometimes become intrusive. As of right now there is no way to disable this feature in the Settings app, but you can stop the handwriting screen from popping up by following the instructions posted below.
You can enable the handwriting feature by holding your device in landscape mode and tapping the handwriting icon on the keyboard. This will bring up the handwriting screen where you can handwrite a note using your finger, or select one of Apple's many handwritten options.
Sometimes when you enable the handwriting option it keeps popping up every time you hold your phone in landscape mode. To stop the handwriting screen from appearing just tap the handwriting icon again to disable the feature.
This is based on the first iOS 10 beta. This FAQ will be updated if Apple updates this feature in the final version.