babies WHO DRINK FORMULA MILK MADE UP WITH TAPWATER are not supposed to drink it made up with softened water as it contains a tiny amount of sodium bicarbonate ( AKA bicarbonate of soda as used in indigestion remedies, baking powder, cakes and biscuits etc) which adds to the sodium in the diet.
It has no effect on BF babies
It should also not be drunk by people who are on a reduced sodium diet, such as some kidney disorders.
Anyone, including toddlers and small children, who eats cornflakes, bread, tomato ketchup, marmite etc gets plenty of sodium from that as they are all packed with salt, so the small amount of bicarbonate of soda which softened water contains is not relevant.