The Shadow DOM specification reads:

"10.5 The shadow HTML element
....
Attributes
    olderShadowRoot of type ShadowRoot
        Represents the shadow tree that is rendered in place of this shadow insertion point
        On getting, the attribute must return a result that is equivalent to running the following steps:
        1. Let TREE be the shadow tree that contains the context object
        2. If TREE does not exist, return null.
"

This text seems to be incorrect.
If TREE (which contains the shadow HTML element) does not exist, the shadow element does not exist too. So it is not possible to read its attribute olderShadowRoot.