This is a lazy post. You can easily find papers on this in google scholar.

See e.g. doi: 10.1210/jc.2006-1375

"A Population-Level Decline in Serum Testosterone Levels in American Men"

There has also been a large decline in sperm counts. The declines seem to have started about mid C20.

At one point, sceptics were quite vocal in their view this was not real but they have gone quiet of late e.g. Professor David Handelsman.

It is interesting that because "normal" levels are based on population samples, the normal levels have been reduced in some places as a result. Levels considered normal now would have been considered seriously low not too long ago.

There is little or no interest in finding out why this is happening. Theories abound.

Estrogen mimics in foods (soy. industrial milk, grains and grain and soy based oils), pesticides, water supplies (excreted estrogen from women taking birth control). My own suspicion is that it is due to a combination of factors

Note in this space be aware that many of the studies have been industry funded and seemingly rigged to produce a desired outcome that product X "has no [statistically] significant effect." Which result is not surprising given that the study was seemingly made so small and of such a short duration that only a huge effect would give the magic p<0.05.

I would add that your narrative that this is a myth propagated by nostalgic macho cowboys is not evidence.

Coming back at this issue it seems possibly quite serious, it certainly seems underexamined.

I found this Vox article and the wikipedia page useful. It seems quite clear that there is quite a large drop in Western countries. That a similar drop isn't seen in non-Western countries is especially telling.

The Vox article also points towards research into in-utero exposure to certain chemicals permanently decreasing sperm count. This also seems quite scary.

For example, there’s compelling data that in utero phthalate exposure is linked to a decrease in something called “anogenital distance,” or AGD, in male babies. AGD is the space between the anus and the genitals, and a man’s is usually twice as long as a woman’s. In men, a shorter AGD has been associated with poorer semen quality, less testosterone, and a higher risk of infertility.

One of the scientists to first describe this phenomenon, Mount Sinai’s Shanna Swan, told Vox that these early chemical exposures have lifelong results. “The lowered androgen and alteration of development that happens in utero [results in] changes that are lifelong,” she said. “They are not correctable. Maybe in the future with genetic modification, you can alter a man’s germ cells that produce his sperm — but for now, if those are impacted adversely, a man will have a lowered sperm count his whole life.”

Separately, a March 2019 study found exposure to phthalates and polychlorinated biphenyl — again, two ingredients found in plastics — damaged the quality of sperm exposed to the chemicals.

This Forbes article is a typical piece about falling testosterone counts in men. Much is made of changing cultural mores, social expectations of men, etc. The biological component seems to get mentioned only in passing.

I found this article by urologists quite frightening. The effect size is eye-popping.

After controlling for confounders—including year of study, age, race, BMI, comorbidity status, alcohol and smoking use, and level of physical activity—total testosterone was lower among men in the later (2011-2016) versus earlier (1999-2000) cycles (P < 0.001). Mean total testosterone decreased from 1999-2000 (605.39 ng/dL), 2003-2004 (567.44 ng/dL), 2011-2012 (424.96 ng/dL), 2013-2014 (431.76 ng/dL), and 2015-2016 (451.22 ng/dL; all P < .0001).