Long (and short) soaking won't remove starch. In my several hay tests of soaked hay, starch wasn't affected at all.
IME, two bad result of long-term soaking are significantly increased iron and decreased phosphorus. I cannot be sure if that varies regionally (based on different soils, different iron compounds, different grasses), but here, soaking makes balancing more difficult because large quantities of phosphorus must be added to the diet while iron ppm increases substantially.
I learned that the only way to keep iron levels near those of dry hay is to quickly but thoroughly rinse in cold water, then soak the rinsed hay the shortest time possible with the hottest water available, say 15 minutes, completely drain away that water and then to do a second soak for the balance of the time. Finally, a thorough, lengthy rinse was done. That's a lot of water in a drought.
Such thorough soaking removes more than sugar: it also removes phosphorus. I had hays that, as a practical matter, couldn't be sufficiently supplemented with phosphorus because the amounts were so high that the supplements were not palatable.
