Patch notes confirmed my suspicion about forums and blind sheep.
"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"
Pretty much said support aren't being graded properly and downvoted to oblivion but then riot themselves say it rofl.