I think
this video helped me understand this issue the most, especially the later parts about foot width.
It also might explain where the reps are getting the 90mm waist width cut-off number from. At13:06 it looks like 92 mm is probably the average foot width for a woman.
I agree w/the video in that the main consideration for an upper limit of the ski width for an in-bounds 50/50 skier should probably foot width (which likely scales with knee width). I'm sure people can comfortably and safely ski something wider if the snow conditions are right or the individual is a stronger skier. Ski shape (amount of sidecut) and length (probably influences leverage to some extent) could also factor in.
I also like this video because it explains exactly what the stresses on the knee are and what knee pain to look out for if you're concerned your skis are too wide. If the boot cuff of your outside knee starts pressing into the medial part of your knee when carving that might be a warning sign.
I hate hard and fast rules when it comes to sports like skiing because everyone has different body types and skill levels, so I like that this video gives some general thoughts on guidelines to follow and what to look out for.
I'm curious about people that are really comfortable with wider skis how the waist width matches up with their foot width, perceived strength and/or ideal snow conditions.