FiveThirtyEight did a piece early this year about a sharp increase in the percentage of batted balls that are foul. That ties in with working the count to make games longer. I don't think we have the historical pitch counts we need to study this (I checked Denny McLain's game log for 1968 on baseball-reference, and nope), but I think it's fair to say 200 total pitches doesn't get you as deep into a game as it once did.
https://fivethirtyeight.com/features...talking-about/