I agree to an extent, but Soraka has never been an issue for me. Honestly, if you can learn to run around with 100 health in a team fight and not die, you'll get an S.
But, I agree that I feel that supports get less help with this grading system. If I run in front of a Jhin bullet to save my ADC and then die as the Akali jumps on me, that means she wasted cooldowns on a support. That gives my ADC time to do stuff. If I play as Blitzcrank and pull a Katrina off my ADC only to die cause I'm in the enemy team, I still did my job. Yeah, if I can do my job without dying, that's better, but it's still something. Catching a Darius mid ult over my 100 health teammate with my Bard ult without clipping my teammate so they can walk away should get me something. That 4 man Crescendo into MF ult should mean something. Even Devouring your teammate as Tahm Kench and spitting them over a wall before you die should mean something. Hell, Phantom Dancer can determine how many times it's saved your life, why aren't saves/sacrifices counted for supports?
I mean, yay, my lulu got three assists on the ADC's triple kill, let's just ignore the fact that the ADC was 1v3 until she flash ulted, shielded, exhausted, and poly-morphed the right people untill he got that triple, those details don't make her assists seem more impressive or anything.