I've done some more research, and it seems that they did exist to a certain extent. It seems to have occured in pre-war kamienicas, and the policy was abandoned in 1956. New build apartments between 1945 and 1990 always had individual bathrooms and kitchens.
Maybe not Poland-centric...but this was also common in pre/early post war flats in the UK. Flats were like rooms with a washbasin and you shared a bathrooom. You can still see it in cheaper B&Bs in Blackpool. Rising Damp was a great British sitcom set in one of these.
I have no issue with no bathroom window, provided there is ventilation. But I really don't get the frosted door glass panel on pretty much every flat bathroom I have been to. While you can't see clearly through, I still don't want to see the outline of someone sitting on the toilet!