Looks like you caught me before my quick clarification

Since you're quite correct in that a number of motherboard manufacturers have enabled VT-d on the Z series chipsets, but that doesn't change the fact that it's not officially supported by Intel. It's a similar situation to overclocking support on the non-Z series chipsets - not officially supported by Intel, but some motherboard manufacturers have still enabled it.
Edit: Just to be clear, the above is my way of saying that yes, Z170 will likely continue the tradition of not officially supporting VT-d... but that doesn't mean that some motherboards won't work around such just like they do with previous Z series chipsets.