Has an NPB (Japan) game ever ended 15-1?
Yes. NPB (Japan) has seen a final score of 15-1 31 times. It first happened on November 7, 1950, when Osaka Tigers beat Kokutetsu Swallows, and most recently on July 1, 2018, when Hiroshima Toyo Carp beat Yokohama DeNA Baystars.
That makes 15-1 the 102nd most common final score out of 223 that have ever occurred in NPB (Japan). One point away, 16-1 has happened 16 times and 15-2 has happened 31 times.