I saw it in Louisiana. For a few months, my high-school cafeteria was giving us milk in a bag. Like individual-size bags that you poked a straw into. I was very confused the first time I saw it
They had them in my elementary school back in the 90s. Haven't seen one since then. They were pretty cool though. I'm sure some people would buy it if they brought it back for nostalgia.
We do actually have the bag and jug too! I love getting my milk at Kwik Trip because that's how they sell it. And it just tastes better. If it weren't so damn expensive, I'd force my household to convert to it.
I used to work in a kitchen of a well-known chain pub in the UK and our mill came in in bags. Saved on alot of waste as we only needed one plastic jug and the bags used significantly less plastic than bottles of milk.
In my experience, it depends on the country and it's only in some stores - I vaguelly remember getting milk like that at some point, which I think it was in The Netherlands.
Mind you, it's more climate friendly than plastic bottles and likely glass (even with recycling).