I have never heard Wallflowers (Erysimum spp., in the Mustard Family, Brassicaceae) claimed as poisonous to cats, even after looking at at several online lists about the topic.
I do know that the main secondary chemistry of the Brassicaceae involves the presence of glucosinolates and the enzyme, myrosinase, which together release potentially noxious chemicals known as mustard oils, when a plant tissues are damaged, such as by herbivory. At high concentrations, these can be toxic to vertebrates, but usually there is only an unpleasant taste or odor, which discourages further herbivory.
Of course, humans have come to enjoy the taste of mustard oils in moderation, as they provide the hot and bitter flavours found in mustard, brussel sprouts, kale, wasabe, horseradish, etc. Furthermore, some health claims have been made about the ingestion of these plant compounds, as well. Exactly how this might apply to the tastes and physiology of cats, I will leave as an open question.