I'm sure you have seen the debates on Safety Kit where plenty of eloquent and experienced people come down on either side of the kit debate.
That would suggest that the change from windproof to waterproof is more symbolic than anything.
You can go and get a £20 waterproof which offers a little protection, or a £150 windproof which may offer more protection.
Runners are given a checklist of what they should carry and aim to tick off that list.
I usually have more than what I am asked to carry - I'm sure others do, but others, particularly the less experiences ones may just carry what is in the rules and think job done.
I ran at Duddon around 4-5 years ago. It was 25C, clear blue skies. June. With limited space in my bumbag would I be better off carrying a water bottle, some nuuns, factor 20 and an extra couple of gels than full body cover?
People were dropping out at Three Shires Stone affected by the heat. From there to the finish I don't remember seeing any water.