For the purposes of this discussion, you really should be looking at more than just the current generation of chips, because the installed base that OS vendors have to worry about includes a lot of processors that are more than 18 months old, and a lot of processors from the budget product lines. The 5th generation parts so far are just ultra-low power tablet and ultrabook CPU, so almost all of the 4th-gen Haswell parts are still current.

Intel's been a lot better about including VT-d on laptop chips, especially recently, and haven't disabled it on the consumer rebrands of their server chips (the "i7 Extreme" parts) in the past few generations, but did on earlier generations. Among the desktop parts, they've been all over the place, and most notably all but two of their flagship overclockable desktop processors (-K models) have had it disabled. Those models have most likely outsold their i5 and i7 counterparts that do have VT-d, and probably themselves been cumulatively outsold by the i3, Pentium, and Celeron processors that also lack VT-d. A raw count of the model numbers shows that in the time since VT-d has been released, the desktop processor models have been split 134 to 63 in favor of not supporting it.

The overall picture is that VT-d support is at least as hit-or-miss as HyperThreading support, which the Steam Hardware Survey finds to be present on about two thirds of Intel machines. Motherboard firmware support for VT-d is even worse, and of those that do support it, it's usually off by default.