Microsoft is steadily adding all the essential features to the new model-driven form designer (preview) making it more and more usable and intuitive.
Latest addition is the support for sub-grid as shown below
The properties window for the sub-grid
We can select the view from either the related entities or view from any of the un-related entity (uncheck Related Records check box).
Similarly, we can enable view selectors and can show all the views or selected view as shown below.
We still can’t add new sub-grid to the form in the new form designer, for that we still need to go back to our classic form designer.
Check out below articles for more details –
Hope it helps..