This is most definitely a practice from a bygone era.
In the late 1800s, “baby airing” started showing up in parenting books. The idea was that small, cramped apartments in big cities were harming babies, and fresh air was beneficial to little ones. In his 1894 book The Care and Feeding of Children, Dr. Luther Emmett Holt said, “Fresh air is required to renew and purify the blood, and this is just as necessary for health and growth as proper food.”
Luther’s call for fresh air for babies led to the creation of the portable baby cage in the 1920s.
The woman who applied for the patent on the baby cage, Emma Read, argued: “it is well known that a great many difficulties rise in raising and properly housing babies and small children in crowded cities, that is to say from the health viewpoint.”
Baby cages really took off in cities in the 1930s, particularly London.
The trend stuck around for a while, as this video from 1953 shows.
It’s not entirely clear when the baby cage fad went away, but I think it’s safe to say that, with overprotective parents hovering over their kids’ every move, levitating baby cages out your window would not fly today. Maybe they’ll come back in style one day? I’d say the odds don’t look good.
h/t: Mental Floss