This enhancement falls under small but powerful (and frequently requested from customers).
Previously multiple object style subcategories could be selected, but not deleted. You still had to delete them one by one (which could be problematic if you had hundreds or thousands you wanted to cleanup).
With the release of Revit 2016 R2 it is now possible to both select multiple object style subcategories and delete them:
It is important to keep in mind the underlying logic has not changed (for example you still can’t delete hard-coded object style subcategories). If you select some subcategories you can delete and some you cannot - the option to delete will still be unavailable. Revit will still warn on attempted deletion - now giving a total count for multiple subcategories.
But for user or family created object styles, or for all of those imported object subcategories you want to quickly cleanup - this could save some serious clicks. This functionality is available in both the project and family environment.