The last blog posts about KDM/LightDM/SDDM/WhateverDM left things a bit on an exciting cliffhanger so I've been asked a few times what the current state is.
The short summary is we recommend SDDM as the display manager for Plasma 5.
KDM was dropped from Plasma 5. KDM includes code from XDM dating back to 1988! It had served it's job well. However, we're now at a point where we need the backend to be Wayland ready and we want to use more modern QML in the front end. When you have to replace both the back and front ends, it's a sign to just start from scratch.
There was some work done 2 years ago into sharing code with LightDM. In the meantime a separate project was started, SDDM which is (yet another) display manager.
Although personally I was very happy with what we had with LightDM it definitely doesn't make sense to split resources, so we focussed everything on SDDM and I have been helping work on that transferring knowledge from my old project.
SDDM is itself in Github and usage is shared with other desktop enviroments, in particular Maui. This makes it harder to add Plasma integration in the code itself, but we've managed nonetheless.
The visual design group made some mockups of a new display manager, which became the basis for the implementation of our login screen.
This is shipped with plasma-workspace. It is up to distributions to update /etc/sddm.conf to update the default theme on installation as it's not something we can do from our code.
In addition we also provide a configuration module that fits into SystemSettings. This is now back in the KDE repositories and will be included in Plasma 5.2.
Naturally Plasma can still work with any display manager as half of the point of having a display manager is to allow choosing which session to launch.