if riding a mountain bike has taught me anything, it's that you don't really want all that much vertical compliance in the frame unless you are hitting big rocks. I know people that use suspension seat posts on their gravel bikes, which seems like a good solution. I have never really wanted a road bike to have more vertical compliance in the rear triangle. The fork, maybe, but the best solution for that in my mind is bigger tires. The whole "horizontally stiff, vertically compliant" thing seems to have been questionable in the first place. Lots of people like a bit of spring in the horizontal direction, describing very stiff bikes as feeling dead.