I knew I had seen this option somewhere at some point in KDE's plethora of customization options. But why is it in the keyboard advanced options??? Shouldn't this be in the display config section? Or at least in the shortcuts section!
In unix terminology, "display server" refers to the user-facing input and output devices. That typically means the keyboard, video display, and mouse. I'm glad they didn't stuff all those words into its name, or call it something cryptic and confusing like "KVM server". ;)
In any case, I think putting an optional key combination in the keyboard options makes at least a little sense. I would guess that it didn't end up in the shortcuts section because it's not implemented by the same component(s) as those shortcuts. Control+Alt+Backspace to kill the display server has historically been a lower-level feature, not even requiring a desktop environment at all.