If a unit gains Deflect during a reaction window, does the controller of a spell targeting that unit have to pay the Deflection cost?

Ruling: No. Targets are declared the moment an ability is put on the chain, and costs are paid at that time. By the time you can react and give Deflect to a unit, it has already been targeted and costs have already been paid.

Nuances:

• For cards that create reflexive triggers when resolving (like Falling Star and Icathian Rain), you could give something Deflect by reacting to the ability that creates the reflexive triggers, before targets are selected for those triggers.