Supports (well actually not only them) have shields, heals, cc, like many tools. But debuffs are so unexploited. The only spells i know with it are 
and Sona's W's powerchord.
I know Riot feels like they are toxic to the game but I don't see why. I feel like a very short but more or less powerful debuff is a much more interesting mechanic than a shield or heal.
is broken enough already
Every support gets Exhaust, arguably the most broken summoner in the game that also happens to be a debuff, in addition to hard CC that most supports have (which is still a debuff, just an incredibly powerful debuff)
Do you mean damage reduction debuffs?
to bad, the debuff sucks unless you go full ap
25% damage reduction for 2 seconds for what happens to be the most situational thing ever since you have to get 3 stacks on her passive just when the enemy is going to use his burst
Riot doesn't like debuffs and feels they're invisible power.
