Soy decreases allergenicity when fermented, which might be meaningful if you are allergic or intolerant to soy.
But less well known, some are allergic to fermented soy but okay with soy.
There is a study on late onset anaphylaxis with natto allergic. Unusual in that it is IgE mediated but triggered only when it reaches the intestines, taking a mean of 8 hours.
I am not okay with soy but my reaction is much worse with raw fermented soy. I suspect it is the difference between soy protein intolerance versus mold (aspergillus) allergy. If I didn’t have the latter I might have tolerated small amounts of fermented soy, such as that of a typical condiment serving size, which would have opened up some more flavor adventures, alas.
It was frustrating at times to constantly suspect yet another ingredient, sometimes even obscure additives in processed foods. Many things were eliminated at one point or another, usually something newly added or relatively infrequently eaten. But symptoms still continued. In hindsight, in such a case it is more likely due to something you eat fairly frequently yet never suspected for one reason or another.
The main one was soy. The reason it was not suspected was that a larger amount was needed to cause reactions and the immediate reaction, headache, was relatively subtle (although it sometimes got worse over the next day or so) and tended only to happen with less processed forms of soy such as edamame and soy milk. Other more processed forms of soy like tofu caused headache the next day. Diarrhea was more severe and problematic but it also was usually delayed by a day causing us to suspect many other foods eaten right before the incidents.
Later I learned that soy generally tends to have a much higher sensitivity threshold than other allergens, like 100 times higher. So even if you have soy allergy you are unlikely to react to soybean oil, soy lecithin, even soy sauce.
Perhaps my reaction to soy is not an allergy in itself but rather some kind of cross reactivity to my other allergies. My symptoms are a less severe hybrid of that from nuts/ grains (headache) and milk (diarrhea), and the latter is much delayed.