Prohibiting the use of Dolphin as the actual root user (not using sudo or kdesu) breaks legitimate use cases for using the root user. An example is Kali, a distro that logs in as the root user by default as a deliberate design choice.
In such an environment, there is no additional security vulnerability beyond what you're already potentially exposing yourself to. So, let's re-enable it.
BUG: 387974
FIXED-IN: 18.08.0