It seems almost impossible to get an S as a tank support. I have a couple of mastery level 6s, but even they were extremely difficult to get despite having some seriously awesome games.
Attached is a link to a picture showing my latest game - I went 0/3/25 on Thresh in a game where my team had 46 kills (~55% kill participation). I did 14000 damage, almost as much as the enemy adc and midlaner. I even got 60 friggin' CS, which as a tank supp isn't too shabby.
My rating? A minus. In my view, that's ridiculous. I went 0/3/21 earlier on Nautilus supp and we stomped the game in 25 minutes, and I still didn't get even an S minus... This is a recurring thing; getting an S on a tank supp is way too difficult.
Does anyone know how the hell this rating algorithm works for supports? They really need to revamp it, in my view, because I don't see how I can realistically get an S or S+ as a tank support if 0/3/25 is an A minus. It seems even as a support, kills are required to get a good rating, even if that means stealing them from your carries, and is also very hard to do on tank supports.