A ‘decisive pass’ is a literal translation of the words used in French to mean what English speakers usually use the word ‘assist’ for. I’m not aware of native English speakers using it, but that doesn’t mean it’s impossible.

In this case, "decisive" = "deciding" = "conclusive". Yes, it's an adjective. "Decisive" has another meaning that is not at play here: "decisive" = "resolute" = "determined". This second meaning would be used only as an adjective to describe a person, not a game. example: "He makes decisions quickly. He is decisive."

A "decisive" game is one that will "decide" who moves on in the tournament. The loser is either eliminated completely, or their path toward moving on becomes much more complicated. A "decisive" goal is one that "decides" a game. A pass can't really be decisive, but it can lead to a decisive goal.