yeah, gzdoom cannot just replace it, because some obscure mod or map will inevitably break. lucky me, i don't care: k8vavoom is already too far away from vanilla, and i am not going to support every mod or map out here. ;-)
for others: as Graf said, don't expect it to be in gzdoom engine. introducing this "small" (no, it is not small at all) feature will break alot of things you may not even know about. and then some other people will come complaining that their lovely mod/map is not working anymore.
and no, having two separate coldet implementations won't work too. gzdoom still have occasional glitches with portals, and portals are much less intrusive. adding another independent coldet mechanics will cause chaos all around. plus, now devs will have to support two completely different code bases for the same thing, and this is like x6 harder (no, not x2, at least x6).