From Verducci's Article:
It may seem counterintuitive, but as pitchers throw faster, they throw fewer fastballs. The use of fastballs (not including cutters) was as high as 58.4% in 2009. Last season, it fell below 50% for the first time, to 48.5%. This year, it is down further, to 46.9%. At this rate, hitters will see 80,886 fewer fastballs than they did 14 years ago, even though the average velocity of those fastballs has gone up 1.9 mph.
But what biomechanics have done for velocity, technology has done even more for spin. Breaking pitches have become so nasty that, this year, a run-of-the-mill slider is tougher to hit (.218) than a fastball at 99 mph and above (.232). For example, St. Louis righthander Jordan Hicks has thrown the most 100-mph pitches this year (110). Hicks hit 100 mph more times in April than the entire league did in April just five years ago (92 in 2018). And yet, batters hit .333 against his triple-digit fastballs.