Has an NPB (Japan) game ever ended 15-5?
Yes. NPB (Japan) has seen a final score of 15-5 25 times. It first happened on May 22, 1955, when Nishitetsu Lions beat Tombo Unions, and most recently on April 24, 2019, when Hokkaido Nippon-Ham Fighters beat Tohoku Rakuten Golden Eagles.
That makes 15-5 the 108th most common final score out of 223 that have ever occurred in NPB (Japan). One point away, 16-5 has happened 19 times and 15-6 has happened 13 times.