I’m just young enough not to have had this in my school and never went to any YMCAs. But being naked in the locker room at school and the recreation center I swam at was very much a thing.
It seems to have ended at the insistence of mothers, not the young men doing the naked swimming. It started out as a hygiene issue with heavy wool swimsuits being a breeding ground for any number of nasty infections. Then the filter couldn’t handle the fibers in the water. Eventually, it was what it was, and I believe it was healthy.
I feel sorry for young men who never have any healthy group nakedness to allow them to become more self-confident and less hung up on nudity leading to all kinds of body image issues.