To reeanable CTRL-ALT-BKSP (since 11.2), you use:
setxkbmap -option terminate:ctrl_alt_bksp
It works. However if you map keys with xmodmap, it’s gone.
In whatever order you use them, setxkbmap and xmodmap are always conflicting. Running xmodmap immediately disables CTRL-ALT-BKSP, while running setxbbmap reverses key mappings to their default.
This is an old and recurrent problem and I really doubt there is a solution … but maybe somebody has a hack or an idea ?
AFAIK they cannot be used together. But, I get the idea you still have an xorg.conf…Rename or remove that, and configure your keyboard behaviour through Systemsettings - Country Region etc. The various keyboard options can be set there, per user !!!
It wouldn’t work for my xmodmap needs :
remap Caps_Lock to Mode_switch and use it in combination with all other keys (in state 3 and 4 ) to type locale specific characters on my US keyboard, as well as remapping some extra keys on different keyboard models. Plus I use the same Xmodmap under 4 Linux distros and 4 BSD OS on a dozen machines and about 10 different wms. Per user settings are not reasonable and won’t do what I want anyway.
I came with that old problem today because I never had to use setxkbmap before and now, it appears to be the only solution to restore CTRL-ALT-BKSP functionality. 
I seems like we’re having more issues with xmodmap. At some point, all key mappings are gone, apparently for no reason, as if it were a hidden daemon which would reset all keycodes to normal. I noticed since the Caps_lock was suddenly back to its original function >:(
Right now, I just experienced the problem on openSUSE with KDE but I’m quite sure I’ve observed this behaviour on other vms and/or other Linux. Plus my wife complained today she lost her xmodmap keys, while she was typing under Ubuntu/Gnome. So it looks like a xmodmap/Xorg problem.
I guess we should probably not use xmodmap anymore. But I don’t know what else could remap keys at such a low level as I need (I mean not on a per user or per wm base, even not on a per OS base).