There's pretty much always been offside - the offside law was relaxed from three to two players in 1925 or 1926, I think, though.
Andy Cole has the highest single season Premier League tally of 34 goals in 40 games in the 93/94 season....
Shearer hit 34 as well the following season (albeit in 42 games) and a slightly more impressive 31 in 35 the year after that.