I guess the same will prove true for 96.43.20. I tried running with -ignoreABI, but that only made the server segfault, so the workaround for now is to block the 1.11 upgrade using something like:
echo >x11-base/xorg-server-1.11 >> /etc/portage/package.mask/nvidia-drivers
I'm not even sure it should block the stabilisation bug.
Oh, I should, I guess, since the Whiteboard on the blocked bug says:
if you find regression please DON'T COMMENT HERE, instead
file a new bug and make it block this bug
I guess similar to bug 375215, NVidia will take more time to update legacy drivers.
If I understand correctly, no package.mask is necessary, as the <x11-base/xorg-server-1.11 dependency will make 96.* and 173.* users skip the xorg-server upgrade.
If package.mask is preferred, http://www.gentoo.org/doc/en/nvidia-guide.xml could tell users to additionally mask >=x11-base/xorg-server-1.11 for the legacy drivers.
The depends present are correct to make users skip the xorg-server update.