Currently, in the Timeline Navigator, it is possible using the Shift+Click keyboard shortcut to 'multi-select' multiple Elements at once.

However, since this seems to only multi-select the Elements in the Timeline and not in the Navigator, many of the beneficial features of Navigator multi-selection are not available to use.

This includes for example, multi-selecting a collection of Elements then pressing 'Delete' not working, or multi-selecting a collection of Elements, then not being able to change multiple of their Properties-in-common at once.

See attached video.

Repro Steps:

1) Create a new Studio Project featuring a MainPage.xaml

2) Introduce at least 3 different Elements into the root Grid of the Page.

3) Navigate to the Animation Panel's 'Storyboard' tab, and create a new Storyboard.
This will open the Storyboard for edit, with 'Recording Mode' automatically activated.

4) With 'Recording Mode' active, key any Property changes on the Timeline to the Elements introduced in Step 2).
This will populate the Timeline Navigator with the Elements.

5) Hold [Shift] and click the top-most Element in the Timeline Navigator, then, without releasing [Shift], click the bottom-most Element in the Timeline Navigator.
This will produce a multi-selection of Elements in the Timeline Navigator.

6) Press the [Del] key to delete all Elements in the multi-selection.
Observe how only one of the Elements is deleted from the Timeline.

7) Repeat Step 5) once again, multi-selecting all Elements.

8) With the multi-selection active, in the Properties Panel, change the Opacity value.
Observe how only one of the Elements in the multi-selection has had its new Opacity value keyed.

Expected Results:
Multi-selection features function in the Timeline
OR
Multi-selection can't be performed in the Timeline Navigator.