http://www.techradar.com/news/a-new-apple-pencil-for-the-iphone-could-arrive-in-2019

It seems a certainty that Apple would at least be thinking about adding support for the Apple Pencil to the iPhone, but exactly when it'll happen (if it ever does) isn't so clear. A new report puts the date at 2019, so perhaps just in time for the iPhone XII.

The new report comes courtesy of The Investor in South Korea, which says its sources reckon an iPhone with a "digital pen" is going to appear in a couple of years time. Apparently this would use a slightly different technology to the current Pencil, so you wouldn't be able to use your iPad stylus on your iPhone.