I am going to say this again.

An XP mastery for supports could not feasibly work in the current (IMO trashy) mastery setup. If it was a keystone mastery all other support masteries would become useless. Who needs 10 bonues AR/MR when you have a level advantage? If it was not a keystone mastery then non supports would end up taking it in case they got behind.

The only reason why it worked in previous seasons was because of a dedicated support mastery tree. Other champions would have to sacrifice too much if they wanted the mastery in case they got behind, but because of the current mastery setup you barely have to commit to a mastery tree.