As someone else mentioned, Duet Display also ceased to work with 10.13.4
However, they have issued a beta driver that is working for me, and they indicate a stable driver is expected in a few weeks.
So I'm back up and running, extending my MacBook Pro 13" display onto my iPad Pro 12.9" with Duet Display.
An interesting thing I had not noticed before - they seem to be using AirPlay to do this. I don't think that was the case before.
While connected, the Displays icon shows in the menu bar and is blue - clicking it shows an active AirPlay connection.
I don't know if Displaylink could do something similar or not.
I've attached screenshots of the menu bar, Displays arrangement, and iPad Displays screen.