What costs does Ezreal, Prodigy reduce? Does he reduce Repeat costs? Accelerate costs? Equip costs?

Ezreal, Prodigy reduces additional costs that are optional to pay. These will generally use both the words “additional cost” and the word “may.” Repeat and Accelerate are the two main sources of optional additional costs, but Ezreal can also offer a discount for one-off oddballs like Blast Corps Cadet. Ezreal can’t reduce Equip costs. That’s an activation cost for an ability, so it’s neither optional nor additional. There aren’t currently any optional additional costs that apply to activating abilities.