Supports don't notice toxicity more, but being a support does in fact indirectly lead to toxicity elsewhere, in some respect. Here's why.
You have 5 roles, 4 of which people believe are the hardcore "omg I have to kill everything to win or I lose game ggff@20." Supports generally don't fall into this mindset because they help the team by offering vision and setting up and engaging to speed the ADC along. They play more carefully and sometimes passively, and their general risk as compared to other lanes is a little lower because they always have back up, and their death does not mean a distinct lane advantage for the enemy in a lot of cases.
So with that information, this can generally upset other roles when they die, because their disadvantage of being dead is felt a little more than when the support ends up dead. Jungler loses map pressure and risks getting counter-jungled. Top, Mid, and Bot lose CS and also can give up bounty gold if they've got kills. This kind of temporary setback can cause people to tilt, and thus, become toxic if it becomes a thing. This is why playing as a support leads to more chances to encounter toxicity in other lanes, as there are now 4/5 chances to encounter players who might tilt off the planet and start raging. But if, say, you decide to play mid, and you know you don't tilt or get toxic, this means that you will generally now have a 3/5 chance to bump into other toxic players, because supports don't typically have the self-perceived stress and pressure on them that will cause them to become toxic. So yeah, that's my thoughts on the matter.