Several patents spotted by Patently Apple suggests that Apple is testing the Apple Pencil with the iPhone. The patent shows the Apple Pencil being used with apps such as iMovie.
Personally, I would like to see Apple expand on the iPad before implementing support on the iPhone. That goes for both of Apple’s ‘pro’ accessories for the iPad Pro. However, the patent above also includes demos being used in iMessage, possibly implementing it with the new handwriting feature found in iOS 10.
As always, take the patent with a grain of salt. As a patent from Apple does not always mean it will make it to a product any time soon, if ever. However, this does signal that Apple is looking to implement support for the Apple Pencil in more apps, at the very least.