I’d be interested in the answer to this too. I’ve been tucking almost every workday for the past half decade since I first learned about how to do it. I have not noticed any effects that I have read might be associated with lower T but have never had sperm count tested.

A warm testicle held too close to the body increases the risk of becoming unhealthy (potentially growing a tumor). If it's too tight, you cannot feel it for lumps. These are things my urologist told me. I doubt that a urologist will help you with this idea. Mine were often retracted (usually staying inside me), which was the justification for insurance covering my castration. That is why a scrotum is part of the male anatomy (to keep the testicle cooler than the body). And when it gets too cold for the testicle, the scrotum skin pulls the testicle up tight to the body.