The real point is no one is using HBM1 0r HBM2 on a gaming card, not even a Pascal Titan X.
Maybe this is to do with price or maybe it is to do with performance. Whichever way you want to look at it both AMD and NVidia have decided to use other solutions for their gaming cards.
Nobody sensible is using HBM because its limited to 4Gb so no good for hgh end cards and for low end cards, they dont need the bandwidth or the power saving so pointless either way.
HBM2 is expensive and as it turned out the bandwidth wasn't needed on Pascal, neither was the power reduction.
Some may argue against that to a degree since Pascal gpus are more limited by hitting their power limits rather than heat as fair as overclocking goes.
With Nvidia's next card you are supposed to have between 16Gb and 32Gb and with the bandwidth needed, gddr5x would just take up too much power to give you the bandwidth so you dont cripple your gpu.