To answer that question, I went to the Play Index feature of Baseball-Reference.com. I was able to find (thanks to a paid subscription to Play Index) the number of players, by season, with an OPS+ statistic* of 150 or more, from 1901 through 2009. Dividing each season's number by the number of major-league teams, I obtained the following result:
Observations:
1. There has been a slight but noticeable decline in the average number of players per team with an OPS+ of 150 or more, especially following the second round of expansion in 1969.
2. The surge from 1996 to 2002 probably marks the peak use of performance-enhancing drugs.
3. The decline resumed after 2002.
Thus, for whatever reason(s), slugging seems to be in decline.
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* Definition: "OPS+ is OPS [on-base plus slugging percentage] adjusted for the park and the league in which the player played," where the league average for a given year is 100. Thus "An OPS+ of 150 or more is excellent and 125 very good, while an OPS+ of 75 or below is poor."
