Side effects of specific drugs are reported and end up in the leaflets when there is causation not merely correlation or coincidence. They don't count all deaths or all digestive disturbances regardless of the cause within that 5%, they pick their 'subjects' for the initial studies carefully to weed out other reasons (eg. alcohol, other medications, comorbidities), studies are double blind and there is a control group using a placebo. None of that is being done here.
You are wanting a report of something in the handbook that has not been studied, could be down to either technique or anyone one of numerous ingredients. It's like putting frightening health warnings on ALL foodstuffs about food allergies and sensitivities regardless of the ingredients, anyone can react to anything at any time it's up to us to be mindful. Most of us are shoving numerous chemicals onto and into our bodies on a daily basis with no understanding of what they are or how they interact.
I do think it's very valuable to have threads like your other one so the issue can be discussed, but the title of this one is scaremongering rather than informative. If you later discover you have a completely unrelated reason for your hair loss OR an allergy to a protein or plant extract in the conditioner you selected then how are you going to take back your WARNING about co-washing?