Is it common for that to happen?
Can retired pros literally not produce any testosterone naturally?
Depends. If they've been on a long time their testes shut down permanently or it can take years to regain normal function.
Even people on TRT lose their testes function over time. They are pretty much relegated to having to stay on TRT for life or be eunuchs.
The body seeks equilibrium. Taking steroids/test causes your body to lower its own production to bring the body into normal range.
This is why years ago bodybuilder only did a cycle or two each year. Going off allowed their testes to resume normal function.
Nowadays the idjits stay on year round which kills their testes permanently.
So, avoid the drugs and remain a man and not a eunuch.