Comment # 3 on bug 1082713 from Terje J. Hanssen

Ok, ssdm is the name of the login menu that remains to the Plasma desktop.

I just tell how this has changed vs previous experience/distributions.

Before Leap 15 when one selected Gnome as default system desktop during the
installation, it resulted in GDM login menu with Gnome as default desktop
session (the latter is user change-able at login of course), also when the
KDE/Plasma software pattern was added during the initial installation (and vice
versa).

Now with Leap 15 the same selections result in and boot up the 'ssdm' login
menu and Plasma as the default desktop session (the latter user changeable at
login).
This happends even if the software/package list before starting the final
installation shows 
System desktop is Gnome
followed by 
+ Gnome pattern
+ KDE/Plasma pattern

As I mentioned, to get and preserve the GDM login menu, I had to do the
installation i two-steps, Gnome desktop only in the initial installation, and
in a second installation step afterwars add the Plasma pattern.