Thats a very controversially discussed topic. There is some evidence demonstrating that levels declined and others demonstrating no decline.
Personally I beliefe there is/has been a decline ongoing on a population level over generations, but primarily due to increasing rates of obesity and a sedentary lifestyle. I also strongly believe that endocrine disruptors play a role or will be playing a stronger role in the future. The extent of which they contribute is at the moment impossible to quantify.
If I had to speculate than I would say that T levels on average peaked between the 1960s and 1980s. Before this period many people were short on nutrients because of different reasons. With the 1980s the obesity epidemic started to pick up and we as a society became more and more sedentary leading to lowered levels on average.
Anyways, there is good data available that the average T levels of young, lean and healthy men did not decline between 1980s and now. So at least this constant discussion that T levels of 1500 ng/dl were normal 30 years ago is nonsense.