The same can be said about soy, and alfalfa, and corn, and oats. Finding soy-free isn’t toooooo hard these days, but finding things without alfalfa is a lot harder. Molasses too.
The issue is that the vast, vast majority of horses are just not allergic to those ingredients. Of the common allergens, flax is not high on the list. The only place flax is a “common” allergen, is Nextmune, which is flawed in general (blood testing not reliable), and extra flawed in how may horses they tag as “allergic” to flax
A huge issue is that the vast majority of feeds are higher in Omega 6 than 3. Removing flax creates a bigger imbalance. Adding flax to the diet isn’t necessarily simple - lots of boarding situations are a one-scoop-shop, no supplements. Or you pay extra.
All I’m saying in this is that Tribute is really marketing to a niche, which is fine, even if it’s not grounded in science. If people continue to buy, great, they found a great marketing spot. If flax were such a common allergen, I feel it would be much more apparent by now, given how many yearrrrrs flax has been in feeds