Why does this not exist? In any ski review or marketing info skis are always just 'soft' or 'stiff' but there is never an actual value given to flex. It's always based on someone's opinion. If it's a ski racer reviewing a stiff park ski they'd probably say it's a soft ski, but someone who skis a buttery park ski would probably say your average all mountain ski is a stiff ski.
I propose reviewers take a few minutes and take an actual measurement of flex. It doesn't need to be complicated. It could be as simple as: a ski flex of 40 means it takes 40 lbs to flex the ski 1 inch when blocks are placed 3 feet apart centred on true centre. Of course this doesn't take into account changes in flex pattern over the ski, but it's a good starting point.
Or take a measurement for the whole ski and normalize for length. Or many flex points along the ski to make a flex curve (I bet ski makers do this).
It would be super useful information to go alongside dimensions and turn radius.