I went thru game files and found that spawn rates are based solely on the level of Motherbase. When the combined level of your motherbase is 280 then S ranks appear on the map. Higher it goes the higher rank soldiers you'll see.
E.g. At Motherbase Lv.280 you'll get 5% C troops, %10 B, 20% A, 40% A+, 20% A++, %5 S in the field. It caps out around Lv.686 with 3% C, 6% B,11% A, 15% A+,35% A++, 30% S.
All Heroism does is increase your chances to get the highest rank soldier you've unlocked based on your Motherbase level as volunteers occasionally and it caps at 500k, past that it does nothing.