For example any grain and legume contain some B-vitamins, potassium and etc. Does nutrients leach out into soaking water? I’m mainly concerned about potassium content and to what extent it is reduced?
Yes, it does, but in return you reduce lectins, antinutrients and makes grains faster to cook and absorbed. So the longer you cook, the more vitamin is destroyed.
How much soaking is important. Vietnamese people used to wash (not soak) rice too much so it stripped B12 and led to some health issues. This is after the germ and brand are removed.
People (government) figured it out real quick and intervened but the bad habit still persists in some families. Nowadays, some people still like super white rice instead of darker color ones.
Beans doesn’t seem to have a problem soaking overnight because it reduces a lot of cooking time.