Looking For A Soy Flour Substitution
It's a bean flour, so substitute another bean flour - chickpea/garbanzo is perhaps the most commonly available one, IME.

Beans bring protein, fiber and fat to the recipe. Soy has considerably more fat (oil) than chickpeas, so you might need to add some oil (or higher-oil things like nut or sunflower seed butters.)

"Fermented soybean powder" is available, and would presumably be more or less equivalent to soy flour.