What they've got now on the Framework is just "arrow keys as Apple did it": they used this full-height/half-height combo for a while but have since returned to the "inverted T" with all half-height as you describe.

I think the sense that space is wasted by those deadzones above the left and right keys is a powerful motivator of these "hybrid" keyboard designs. I do prefer to have them all half-height if any of them are, as it does make it easier for me to "touch type" the arrows.