Interesting comments about frame and wheel stiffness by Damon Rinard in the attached.

I can only imagine the vertical compliance comfort of a newer bike with seat stays attached halfway down the seat tube and a flexible seat post. Tour Magazin is the only one I know who systematically measures vertical and horizontal deflections. Minimizing vertical deflection is important because human tissue does not plane, it absorbs the energy and this also contributes to fatigue.

https://cyclingtips.com/2018/04/jra-...-still-matter/