Has a World Cup game ever ended 15-0?
Exactly once. On September 3, 2015, Qatar beat Bhutan 15-0, and no World Cup game before or since has ended that way. When it happened it was a scorigami, the first 15-0 in league history.
That makes 15-0 the 43rd most common final score out of 53 that have ever occurred in World Cup. One point away, 16-0 has happened 1 times and 15-1 has happened never.