Apple - How do I turn off drawing mode in iOS 10's Messages?

To Turn off handwriting in landscape mode go turn your phone to the side to activate it. Then tap on the keyboard in the bottom right. This should keep the iPhone to a keyboard when you go to landscape.

You can just delete the default Apple keyboard in the settings in favor of a different one. For example, you could download Google's keyboard Gboard, then go into the Keyboard settings, and delete the only labeled "English".

Hey presto, now you only have the Google keyboard, and it won't bring up the drawing mode whatsoever.