There will never be a gold/xp mastery for supports.

I'm sorry, but their won't be. I'd love one for the games where i get support instead of jungle (aka 75% of the time), but it wouldn't work.

if it was a non keystone mastery

This would not work. It would either -end up on every champion that can afford to go down that mastery tree or force champions that don't usually build down that tree to go down it anyway. (E.G. ADCs going down the resolve/cunning tree depending on where it would be placed just for the xp) -be so worthless that not even supports would take it

if it was a keystone mastery

It would far outclass all other support masteries if allowed to be powerful or would be u Why would you every take windspeakers when it means being down 2-3 levels from the enemies? Why would you take 6% damage reduction if it meant being down in levels?

it's a grand idea but it will never work. Not in the current mastery setup at least. The old xp mastery was balanced because it forced you to commit to the support tree, but the support tree no longer exists.