To get S and S+ as a support, i only get it when the following conditions are met.
You need to have a good KDA
You must ward a lot, and clear a lot of wards.
Assisting in tower kills and other objectives.
But i also think it compares you to to the stats of other players in the game. I usually get a S+ on soraka when i have highest KDA in game, wards a lot more then every other player on the map, and are involved in more tower kills and other objectives. AND have healed like 40- 60 K, and the highest damage dealer in game have like 30 k damage. I have some good games i feel should be at least an S KDA wise, but i guess i have done mediocre on other things like warding and objectives, and tanking or healing. IDK what counts the most.