Definitely a higher percentage of behinds the further you go back. But there is the Loch Ness monster of determining accuracy; the "rushed" behind. Not defined in the laws of game, not historically recorded in goal umpire's reports and only relatively recently recorded in the media and by club and external stats collectors. Nor is it clear-cut what a "rushed" behind is and how it has been defined in different eras.In its simplest form, you could.
Obviously there's other factors to take into account such as OOF kicks, position of where the kicks were taken from etc. But since that data isn't readily available, and also considering that OOF is a relatively recent introduction to the game, goals as a proportion of scoring shots is a good start.
Looking at total behinds without knowing how they were scored and the further you go back, no way of knowing, the figures for the AFL using this formula (goals *100) / goals+behinds) are:
2003-2012: 53.28%
1993-2002: 53.34%
1983-1992: 52.02%
1973-1982: 50.24%
1963-1972: 48.21%
1953-1962: 45.83%
1943-1952: 46.99%
1933-1942: 48.16%
1923-1932: 45.97%
1913-1922: 43.37%
1903-1912: 41.87%
1897-1902: 39.30%
Percentage of scores with more goals than behinds:
(lower scoring tends to increase the percentage.)
2003-2012: 58.12%
1993-2002: 58.33%
1983-1992: 53.81%
1973-1982: 47.86%
1963-1972: 39.13%
1953-1962: 30.60%
1943-1952: 34.33%
1933-1942: 40.02%
1923-1932: 29.77%
1913-1922: 22.02%
1903-1912: 21.48%
1897-1902: 17.51%
Percentage of scores with 1.5 times as many goals as behinds.
(lower scoring tends to increase the percentage.)
2003-2012: 26.87%
1993-2002: 27.28%
1983-1992: 22.12%
1973-1982: 17.45%
1963-1972: 14.32%
1953-1962: 10.53%
1943-1952: 12.37%
1933-1942: 14.15%
1923-1932: 10.98%
1913-1922: 6.82%
1903-1912: 8.98%
1897-1902: 9.06%
Besides kicking skills there are factors such as playing tactics, rule changes, weather conditions, ground maintenance and the relatively recent innovation of having more than one ball for a match.