When you're looking for that S...

The grading system compares your CS on that champion in that role in comparison to the average that champion gets at the role. (because nobody plays support kha'zix, its easy for him to have 0 CS and get an S in bot lane, but zyra on the other hand, there are many and sometimes they wave clear and get a lot of CS)

"No CS no S supports" aren't really a thing since you'd still be just like every other average support, low on farm.

Usually as support the biggest factors is the damage you do, the number of wards you've played, the number of wards you cleared with a sweeper, and of course your KDA.