While there's a iOS 10, there's one feature that seems to draw the ire of nearly all users: the Handwriting canvas.on
Thankfully, there's a couple of methods to hide this annoying feature that randomly appears.
The quick fix
Rotate your iOS device into landscape orientation, purposely triggering the Handwriting feature. Only instead of scribbling a ton of nonsense on your screen or cussing at your phone, tap on the keyboard button in the bottom-right corner.
The Handwriting canvas will be replaced by the iOS keyboard. Now, when you rotate your phone the keyboard will continue to show up.
I've used this in the past, and have yet to figure out why, but the Handwriting canvas randomly comes back. When it happens to you, just tap on the keyboard icon again.
A third-party keyboard will get rid of the Handwriting feature once and for all.
With a keyboard app installed on your iOS device, open Settings > General > Keyboard > Keyboards > Add New Keyboard and select the name of the keyboard from the list.