To be honest the grading system isn't very good at grading supports for anything in the first place.
While every other role is supposed to prioritize kills over assists, supports prioritize assists over kills, and assists are worth less than kills. Last time I got an S on a support I had to get 40 assists and the only reason I got that much was because the game went on for a very very long time.
I believe warding does contribute to it, but I'm still warding the most and destroying the most wards on my team. I get sightstone ever game, I always use up my charges before going back to the shop. Warding seems to not contribute much at all, which is detrimental to supports because it makes up a big part of the role.
Gold contributes too, and supports are hurt in this category also. Supports do not earn as much gold usually as the rest of the team in general (although depending on how your teammates did you might have more than one or two). Supports do have ways of getting gold as far as support items go, and I will take cs later in the game or when my adc is not in lane, but it I still don't earn as much as my other teammates especially if they are fed.
I usually do a good job at supporting, after all, it is my main role. I have gotten many of my adc's S roles because I helped get them fed so they could fend for themselves most of the time....yet I never get an S.
One recent game I went 1/0/6, my Jhin adc who I had some REALLY great saves on, went 5/0/5. I believe my adc got an S- and I got an A. We both played our roles perfectly, and the game only lasted 20 minutes since the enemy forfeited, yet the system does not grade supports accurately. The support role is played much differently than the other as far as k/d/a goes, so supports generally do not earn S ranks because the system is looking for kills more than assists.
Another game, I go 2/1/11, my adc goes 9/0/3. He gets an S rank, meanwhile I get an A again even though I was the one who got him kills and kept him alive. He got fed at the beginning because of me, the rest he did himself. The system doesn't recognize that I played my role correctly, because I'm not fed in kills. Game lasted 24 minutes, I had the least gold but the most wards placed out of everyone that game, with a total of 15 wards.
KDA does play a factor in grading, but supports are not played as the other roles are and can not be graded the same. It will not be accurate. I will not have the largest amount of gold since I am the only one in game not cs'ing for half the time, I will not be fed in kills unless I ks my team, and that goes directly against how my role is played. My job is to ward, which I do, to save my teammates and keep them safe, and to secure kills for teammates.
I am working on getting assists, not on feeding myself. My job is to feed others, and because of it I do not earn S ranks.
