Shift-Backspace kills X?

[walt]

I often type Shift-Backspace by mistake, and I must have been doing
it since I bought my current keyboard a year ago or more.

Just lately I've found that Shift-Backspace terminates my X session
just the way Ctrl-Alt-Backspace did in the old days.  This was an
unpleasant surprise, as it's purely unintentional.

I finally traced this strange behavior to a very old setting in my
gnome desktop config files that effectively adds three XkbOptions:

grp	grp:alts_toggle
terminate	terminate:ctrl_alt_bksp
caps	caps:none

I must have copy/pasted these items from somewhere a long time ago
when trying to restore the old Ctrl-Alt-Backspace behavior, but I
can't remember where I got them.

Anyone understand how/why those old options are now being translated
into to my current unwanted behavior?

I'm running gentoo, which frequently updates all sorts of xorg-
related packages and drivers, so it might have been any of them,
dunno for sure.

Any hints much appreciated :)