I think S's on supports have been harder since 6.14
I haven't been able to get a single S or S- on a support since 6.14.
I'm not sure how rito calculates this but its just dumb. I placed 21 wards and destroyed 7, helped take so many towers, utilized my active items in team fights, absorbed skill shots for our back line, set up for every dragon and overall helped carry the team.
I feel like since the "supports S rank" were made "easier" its been much harder to get an S- let alone an S or S+.
6.14 patch notes
http://na.leagueoflegends.com/en/news/game-updates/patch/patch-614-notes#patch-champion-mastery
Mastery grade calculations for supports will be adjusted during this patch. " Since we introduced mastery levels 6 & 7, support players felt even more incentivized to snipe kills to pad their stats and improve their chance at an S or S+. We’re re-tuning the system, sorting out some additional variables so supports get more credit for support-y game-winning plays. "
MEASURE OF A MANATEE Support grades now better reflect relative performance in the role
SWIPER NO SWIPING Support grades specifically weigh assists equal to kills, leaving one fewer excuse for bloodthirsty Flash-Ignites