Reproduction Steps:
1. Open new project
2. Edit > Project Settings > Player
3. Under Icon settings notice that Top Shelf and Wide Top Shelf lets you add additional layers.
Apple does not support layered Top Shelf images since ~tvOS 10 and only icon images can be layered.

https://developer.apple.com/tvos/human-interface-guidelines/icons-and-images/top-shelf-images/

Expected behaviour: Top Shelf and Wide Top Shelf images cannot be layered.
Actual result: Top Shelf and Wide Top Shelf images can be layered.

Reproduced on versions: Unity 2017.1.1p2, 2017.2.0f3, 2017.3.0b4