Cyril Morong has posted some information about “clutch hitting” over the years. He finds that clutch hitting (which he calculates as the difference between batting in close and late situations vs. not close and late situations) has gotten “worse” over time. That is, batters are doing worse in clutch situations; pitchers are doing better.
This is exactly what we should have expected, and it speaks to the increasing use of bullpen specialization over time. Batters haven’t gotten “less clutch.” Pitchers haven’t gotten “more clutch.” But managers have gotten wiser and more aggressive in managing late-inning situations.