Do NSUserActivity-based shortcuts support custom UI?
I've created a custom Siri Shortcuts with the help of a custom Intent. I was able to create both a UI-less version (just Intent) and a version with the custom UI (IntentsUI) which is shown during inline interaction with Siri. Is it possible to have a custom UI with the NSUserActivity-based shortcuts?
No. Apple made it very clear in their WWDC presentation that Siri Shortcuts are divided into two, one using Intents and one using NSUserActivity.
The IntentsUI extension is specifically for those who use the Intents framework.