No outline for an empty layer’s thumbnail in Layers panel
In a previous edition of Ai, when in the layers palette and having a layer with no content, the thumbnail square didn't have an outline. Now it has an outline and leads you to believe there is content there.
Why did you change it? Leave it without an outline so you can delete it without the extra steps of looking for content you might not see (being white) or want to delete.
I feel both Adobe and Apple make changes just to makes changes and justify someone's job. I really resent the accumulation of constant unnecessary changes and this is just another small thing to annoy.