I have seen questions dating back to 2022 requesting this feature and nothing is done, while I see relatively pointless updates to the slice machine coming every week and forcing to update the package for little to no gain.
Developing slices with content relationships without this is one of the points that comes across all the time when listing the problems of Prismic.
Also, debugging slices in production is not any kind of solution.
Any idea of when something like this is going to be possible, is it even on the roadmap?
At the moment real documents can't be selected. It is still a constraint for document links. We have an open feature request for creating a modal that shows an informative message with this information.
The main problem is that not having the option to choose a document of a type hinders greatly the development of slices that contain document relationships, to the point that it might be just better to repeat content in order to guarantee that no broken slice is going live.
