В release-notes грядущей альфы Fedora:
X Server
The key combination Ctrl-Alt-Backspace to kill the X server has been disabled by default as a decision of the upstream Xorg project.
To get this behavior back, add the the line
Option "DontZap" "false"
to the ServerFlags section in xorg.conf.
The Xorg project has changed the default DontZap setting to "true" after complaints from desktop users that accidentally hit Ctrl+Alt+Backspace when trying to type Alt+Backspace, Ctrl+Backspace, or Shift+Backspace, or who had StickyKeys enabled. Ctrl+Alt+Backspace is also a keyboard shortcut for deleting certain expressions in C and Java modes in Emacs.