/usr/share/X11/xorg.conf.d/90-zap.conf is in ubuntu-mate-default-settings 16.04.5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | Section "InputClass"
Identifier "keyboard defaults"
MatchIsKeyboard "on"
Option "XKbOptions" "terminate:ctrl_alt_bksp"
EndSection
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