The sacrifice fly rule:
Beginning with the 1926 season, a more liberal version of the sacrifice fly rule was instituted. The basic provision was that any players who hit fly balls advancing runners to second and third were credited with sacrifices; no time at bat was charged for a sacrifice. After the 1930 season, during which the collective batting average of the major leagues exceeded .290, the sacrifice fly rule was eliminated...
The elimination of the sacrifice fly rule reduced the number of sacrifices from 1317 in 1930 to 789 in 1931 in the National League, and from 1283 to 650 for the same two seasons in the American League. In 1939 the scoring sacrifice fly, exempting a batter from a time at bat when a runner scored after the putout on a fly ball, was restored to the game. This lasted for a single season.
after the 1953 season ... restored the sacrifice fly ... unchanged to the present day (no date on this SABR article) ...
Beginning with the 1954 averages, sacrifice bunts and sacrifice flies were listed separately in the official averages (AL pitching statistics did not separate SF from SH yielded until 1955)... SABR member Pete Palmer has determined that the three sacrifices credited to Ted Williams in 1939 were all SF.
In 1930 Bill Terry:
254/633 = 0.401263823065
If Terry had been charged with just three more at bats:
254/636 = 0.399371069182
No .400 batting average.
In 1939 Joe DiMaggio had his highest batting average: .381. He benefited from the one year restoration of the SF rule. For all other seasons during his career DiMaggio did not have that benefit.
Nor did Ted Williams in 1941 when he batted .406. I think it's been estimated that Williams would have batted .413 in 1941 with the SF rule.
In 1957 at the age of 38 Williams had his second highest batting average:
163/420 = 0.388095238095
In case you were wonderinng Williams had only 2 SF:
163/(420+2) = 0.386255924171
Not much difference.