Quercetin itself should be legal. As a supplement it may depend on which ingredients are added.
Here is some additional info that may be helpful from scders discussing quercetin: "Rutin is the supplement that is helpful with IBD because Rutin is what creates Quercetin in the intestines. If you take straight Quercetin orally and have some trouble absorbing nutrients, it may not get to the intestines where it is needed. "