Hi,
I've just installed the d8 branch using simplytest.me with the required patch to have a look at the most recent changes. When I clicked through the menu points (without changing anything in the standard setup) I noticed that on admin/structure/contact you cannot edit a contact form category. Neither can you use the links provided for 'managing fields' or 'managing display'.
Deleting on the other hand works as expected.
I then created a new category without problems. The new category showed the same behavior.
Can somebody confirm this issue?