Apologies for lack of tables and formatting, as I'm on a crippled laptop.
I did some studying of slugging percentage over the years on Fangraphs. And well here -
The first fraction is number of players who slugged over .500 out of qualified hitters since 1960. The second fraction is .600 or higher, followed by overall percentage. No one slugged .700 until 1994. Jeff Bagwell slugged .750 while playing half his games in the Astrodome which is. . . very admirable.
A couple of obvious notes. 1977 was an expansion year after some really lean hitting years and that at least temporarily ''solved'' some of the problems. 1987 is still an outlier since it came 7 years prior to what seems pretty clearly defined as the steroid era of 1994-2009. In 1999 38 percent of the league was slugging over 500 (30 players slugged higher than Ortiz average over the past two seasons). Coors opened in 1993 which certainly ushered in a huge boost. This year's 9.4 is close to matching the lowest level of slugging since 1992. I really think expansion would boost offense the most, short of reintroducing PED's.
1960 - 13/62, 0/62 = 20.9
1961- 24/85, 7/62 = 28.2
1962 - 16/101, 3/101 = 16
1963 - 12/101, 0 = 12
1964 - 15/94, 2/94 = 15.9
1965 - 13/96 1/96 = 13.5
1966 - 17/96 2/96 = 17.6
1967 - 12/99 1/99 = 12
1968 - 6/89 0 = 6.7
1969 - 19/120 3/120 = 15.8
1970 - 22/114 1/114 = 19.2
1971 - 12/114 2/114 = 10.5
1972 - 12/111 2/111 = 10.8
1973 - 7/115 1/115 = 6
1974 - 8/131 = 6.1
1975 - 13/128 = 10.1
1976 - 7/119 = 5.9
1977 - 31/141 1/141 = 27
1978 - 15/140 1/140 = 10.7
1979 - 22/140 2/140 = 15.7
1980 - 13/125 2/125 = 10.4
1981 - 7/139 1/139 = 5
1982- 21/140 = 15
1983 - 16/131 = 12.2
1984 - 16/120 = 13.3
1985 - 16/137 = 11.6
1986- 10/127 = 7.8
1987 - 31/134 2/134 = 23.1
1988 - 15/130 = 11.5
1989 - 9/126 1/126 = 7.1
1990 - 16/128 = 12.5
1991 - 16/123 = 13
1992 - 12/120 1/120 = 10
1993- 25/150 5/150 = 16.6
1994 - 40/157, 6/150, 3/150 = 25.4
1995 - 35/139, 6/139 = 25.1
1996- 47/147,12/147, 1/147 = 31.9
1997- 39/144, 4/144. 1/144 = 27
1998 - 51/159, 6/159, 1/159 = 32
1999 - 60/157, 9/157, 1/157 = 38.2
2000 - 55/163, 15/163 = 33.7
2001 - 48/156, 10/156,1/156,1/156 = 30.5
2002 - 43/151, 5/156, 1/156 = 28.4
2003 - 48/165, 5/165, 1/165 = 29
2004 - 43/161, 6/161,0, 1/161 = 26.7
2005 - 41/148, 4/148 = 27.7
2006 - 47/160, 7/160 = 29.3
2007 - 38/162, 6/162 = 23.4
2008 - 44/147, 2/147 = 29.9
2009 - 42/154, 2/154 = 27.2
2010 - 27/149, 4/149 = 18.1
2011 - 29/145, 1/145 = 19.4
2012 - 29/143, 1/143 = 20.2
2013 - 16/140, 2/140 = 11.4
2014 - 14/148 = 9.4