A derivative of it (co-written by Larry Yaeger, who worked on the Newton) actually was incorporated into OS X, called Inkwell. You could make it appear as an in

A derivative of it (co-written by Larry Yaeger, who worked on the Newton) actually was incorporated into OS X, called Inkwell. You could make it appear as an input area by plugging in a graphics tablet. It was removed in OS X 10.15. The current macOS/iOS handwriting recognition tech is unrelated.