Complex sentence: Finally, in 1971, Major League Baseball made the use of the batting helmet absolutely mandatory for all batters in the game, however, veteran players had the option of choosing to wear a helmet or not, as they were grandfathered into the rule.
Split into simpler sentences: Finally, in 1971, Major League Baseball made the use of the batting helmet mandatory for all batters in the game. Veteran players, however, had the option of choosing to wear a helmet or not, as they were grandfathered into the rule.