/usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf is in unity-greeter 18.04.0+18.04.20180314.1-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [Seat:*]
greeter-session=unity-greeter
|