This ban has been in place for a long time.
It has been mentioned for a long time that when children eat and go to the pool there is a cut indigestion that can be dangerous. But it is a mistake to say that there is a digestion cut, the correct name is hydrocution and that the problem is not water, nor digestion, but the change in temperature.
The body is hot and suddenly they come into contact with the water, which depending on the time of year can be relatively cold (or not hot enough) and the temperature difference causes such a reaction in the body that respiration is inhibited and blood circulation and overload occurs in the heart that ends in syncope. When happening in the water it is especially dangerous because the child can sink.
So what complicates the fact has nothing to do with digestion and it is important to ensure that there is no sudden change in temperature. Whether they have eaten or not, the entrance to the water, if it is cold, must be progressive. It is recommended to put your feet and go little by little putting your wrists, tummy, etc. Thus, the change in body temperature occurs very gradually and the body becomes habituated without risk.