I believe that if you value yourself as a player, you shouldn't really play support in placement games. While a good support may turn games around, if a good support has a really bad ADC on his side, which may very well happen in placements, no matter how much the support gets it fed, your team banked a lot on someone who can't carry them into the mid to late game.
I'm not that good at the game that much as I took a decently big hiatus to play Pokemon, but from experience as a main support since s4 almost this is what I have noticed.