I know of bug report https://bugs.kde.org/show_bug.cgi?id=81040 , but that one was for kdesktop. I also don't know to which product/component I should report this. It might be a good idea for the xkill keyboard shortcut to display a dialog message saying something along the lines of "You're attempting to kill plasmashell. Are you sure of this?" whenever the desktop is clicked after Ctrl+Alt+Esc. An use-case for this would be when an application misbehaves (memory leak for instance and/or OOM) and the user wants to kill a window with the shortcut and accidentally clicks on the desktop.
it's a useful feature when plasma is frozen.
I dunno, what's the point? Plasma will restart, so it's not a huge deal if you accidentally kill it. If you're messing around with xkill in the first place, you're already in a killin' mood and are aware of the consequences. :)