There are biodegradable soaps and shampoos sold at every camping supply store or website. It just seemed silly and overly dramatic to me to make a warning about toxic shampoo. The original premise of the thread was about showering in a campsite around other campers and I doubt that shower residue would be on the list of most toxic things being dumped on the ground there. I would be a lot more concerned about making a muddy mess in a confined area and having my water drain towards some other persons space.
Toxicity levels are related to quantity as well. One person dumping shampoo on the ground might not have a noticeable effect, but multiple people on a daily basis over years might. If the OP wants to be careful about it, then good on him. The only reason that I weighed in is because I see what sunscreen has done to all the coral around Oahu compared to other pacific islands that don’t have tourists.
By the way, Morro Bay is my old stomping grounds. As teens, we used to have a joke gang sign. Fist and three fingers, three stacks and a rock.