With the release of MacOS Catalina, a new feature called Sidecar has been added that enables an iPad to be used as an extended desktop for a Mac. Also, when paired with an Apple Pencil, it can be used to draw or write in Mac Apps that support the pencil. Since the Apple Pencil (1st Generation) is now supported with the iPad 6th generation, and an iPad Pro is not needed, its use with Sidecar has opened up many more capabilities at a lesser cost.
I’d previously reported on a 3rd party dongle that provided this functionality, which like many other 3rd party solutions that tried to fill this void, now is no longer needed. (https://sites.duke.edu/ddmc/2017/08/29/luna-display/)