Honey mustard is not acceptable for most vegan diets, as its foundational ingredients are, as its name suggests, honey and mustard. According to the book “Mustard: A Global History,” the ancient tradition of pairing mustard with something sweet dates back as far as the 1300s. Since honey is made by bees, though, it may be considered non-vegan (via Healthline).
Less obvious and a little more ambiguous is Dijon mustard, traditionally made with brown mustard seeds and white wine. The origin story of Dijon mustard is a pretty fun one; per the BBC, ancient Romans planted mustard in the same field as their grapevines because mustard plants provide nutritious phosphorus in the soil for the grapes. One fateful day in the 1700s, a Dijon city local in France made the happy experiment of combining mustard seeds with wine juice, and the rest is history. But unfortunately, according to Inside Science, many wines are filtered through agents derived from animals like cows, chickens, and fish — meaning those agents render Dijon mustard a decidedly non-vegan condiment.