Cervelo, and probably many others, have had proportional stack/reach scaling for several years already.
AFIK at least.
Cervelo was the first to promote a constant slope of stack versus reach. The prevailing approach was to scale effective top tube length with seat tube length, but then adjust seat tube and head tube ad-hoc to accommodate front tire clearance and rear tire clearance for constant chain-stay length and fork rake & length. This tends to flatten out the reach in the small sizes (seat moves forward for the spec's TT length) and blow out the trail in small sizes (HTA is slacker with same fork rake). The stack and reach measurements implicitly assume a slammed stem, however. If I go bigger on a given frame I need more spacers for the same stem, or more stem rise, each of which moves the handlebars close to the seat tube since the head-tube is sloped. So if I choose between two frames, one more stack, one less, with the same reach, I still get the handlebars further away with the taller frame at a given handlebar height. The argument is I want the handlebars even further from the bottom bracket with a taller rider, so I then need to achieve that with stem length assuming same spacer height. It seems that can work up to a certain point. The steering moment on the handlebars is a hypotenuse with the sides being effective stem length + reach, and half-width of bars, and the latter is bigger so has more influence then differences in stem length from, for example, 6 cm up to 15 cm (half-width is 18 cm to 23 cm).
In any case, constant slope stack-versus-reach seems generally more carefully thought out. Still, that doesn't mean for a given person in their given size it will yield a better fit. It's all about probabilities, most of the difference likely in the smallest and largest frames.