As the summary says, Shift+Backspace and Shift+Delete have no effect under xkeyboard-config-2.4.1-r1 (and similarly in 2.4.1, which had been removed from portage). With xkeyboard-config-2.3, Shift+Backspace and Shift+Delete had worked as expected (i.e. had the same effect as Backspace and Delete). Upstream bug with xev output filed at https://bugs.freedesktop.org/show_bug.cgi?id=43098 Note: this is not a duplicate of bug #386561 (which was fixed in 2.4.1-r1 and in upstream git).
Patch applied in 2.4.1-r2