The decision to disable/change the venerable Ctrl-Alt-Backspace keyboard shortcut for killing the X server in the Ubuntu/Gnome desktop has been a persistently unwise one.
At least for now, the most straighforward way to restore it is to install gnome-tweaks. Then go to:
Keyboard & Mouse > Additional Layout Options > Key sequence to kill the X server
Ctrl+Alt+Backspace: checked
References:
Levente. "How can I enable Ctrl+Alt+Backspace?", Ask Ubuntu, 21 June 2921, https://askubuntu.com/a/1346330.