Official Riftbound ruling
What about Deflect costs? Aren’t those optional now?
No, Ezreal won’t help with Deflect costs, and Deflect is not optional. Deflect is a mandatory additional cost, meaning you must pay it if you want to finalize a spell or ability that chooses something with Deflect. It’s true that you can choose not to pay Deflect costs for triggered abilities by choosing not to finalize them, but that doesn’t make it an optional cost.
Riot's question and answer, word for word, from theRiftbound Spiritforged FAQ
The card
Ezreal, ProdigyUnit
When you play me, discard 1, then draw 2.
Optional additional costs you pay cost
1 Energy or 
Any Rune less.