No Devo, I don't remember having any kind of sickness in my youngsters back in the 80s.
However speaking from my own experience, I also never had as many pigeons back then and was a lot more selective over which birds I bred from, my whole youngbird team was usually made up of babies from not more than about 6 pairs of pigeons. Taking two rounds from each pair giving me around 24 youngsters, they were weaned into a completely seperate loft from my old birds.
These days, since starting up again, I have far too many birds, young and old all run together. I have been gifted a lot of birds from various sources so felt the need to keep babies from far too many different pairings in my efforts to develop a team, this has led to me keeping birds which if I'm honest I should never have bred and a very overcrowded shed. Although I don't really know what causes YBS, and as you have all said, I'm sure there are a lot of different reasons, I'm convinced that it is stress and overcrowding that is the trigger in my birds. At the moment my babies are doing ok but I'm always on tender hooks waiting to be hit, as over the past few years I have been hit and have lost birds due to it.
I have however found that by vaccinating my babies for PMV at weaning age has definitely made a difference, probably another coincidence but something that I will continue to do.
Again, just my opinion.