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.
Gochujang is a Korean condiment. A mixture of hot pepper, sweetener, and fermented soy paste (doenjang). I had been avoiding these for some time after going gluten-free as most commercial versions contain wheat and/ or barley. I finally found some products made the traditional way that were gluten-free. I made a bowl of bibimbap (rice, egg, and sauteed veggies) and added a dollop of the gochujang. But a short while later I ended up vomiting and sweating profusely. I had been episode free for over a year. I am pretty sure this gochujang was the culprit because I ate everything at home that day, made myself, and everything else were items I knew to be safe. When a reaction is this painful and I’m pretty sure about the culprit, I don’t want to test it again for the sake of knowing with absolute certainty. This incident finally got me thinking about that allergy I had to mold and especially to aspergillus mix. Traditionally fermented soy paste can include many varieties of molds but the main one is aspergillus oryzae. Unlike most commercial versions, the product was also unpasteurized, which meant the cultures were alive, and may have been the critical reason why this was the first clear acute reaction I had to a fermented soy product.