For no reason other than nostalgia, and wanting to see what's up with the distro these days, I tried installing Slackware in VirtualBox. I could start Xfce, and other WM's just fine, but KDE would crash.
I did a little digging and came across this.
Create a file called /etc/X11/xorg.conf.d/disable-composite.conf and in that file place the following:
You should now be able to startx and have a working Plasma desktop.