Currently, there is a bit of a loophole where if a Trigger containing a PlayStoryboardAction is created, a Storyboard Resource can be created from the Trigger's Actions section.

However, this produces no results.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) Navigate to the Interactivity Panel, select any valid Element, and create an EventTrigger for it.

3) With the EventTrigger selected, in its Properties Panel, under the 'Actions' section, create a 'ControlStoryboardAction'
As the first option, a 'Storyboard' Sub-Property will now be visible, featuring a (+) icon on the right of the panel.

4) Click the (+) icon to create a new Storyboard Resource. Name it as desired, and stored in GlobalResources.xaml

Observe how the created Storyboard has not populated the 'Storyboard' Property slot.

5) Navigate to the Resources Panel, and select GlobalResources.xaml

Observe how there is no Storyboard Resource present in the location it was saved to in Step 4).

Expected Results:
Storyboard Resources can't be created as Resources from the TriggerActions, and should not display the (+) Create Storyboard Resource button.
If a Storyboard in the scope has been selected in the TriggerAction, it can however be edited in place via the Pencil icon, and provides the same functionality as that provided by https://www.noesisengine.com/bugs/view.php?id=4928