Everything that is known about Siri Shortcuts is covered in this article by Federico Viticci today on MacStories:
On the surface, Shortcuts the app looks like the full-blown Workflow replacement heavy users of the app have been wishfully imagining for the past year. But there is more going on with Shortcuts than the app alone. Shortcuts the feature, in fact, reveals a fascinating twofold strategy: on one hand, Apple hopes to accelerate third-party Siri integrations by leveraging existing APIs as well as enabling the creation of custom SiriKit Intents; on the other, the company is advancing a new vision of automation through the lens of Siri and proactive assistance from which everyone - not just power users - can reap the benefits.
The article is in-depth, comes from one of the people most knowledgeable about Workflow (the source of Siri Shortcuts) outside of Apple, and has details that suggest that Federico spent some quality time last week in San Jose discussing these topics with people in the know.
If you want to know about Siri Shortcuts in iOS 12, this is a must-read.