About 1880. But I still have a whole bunch of them, I think, that I had collected over the years. Some as long as 6+" and as short as 1/2". They really came in handy when I got into refinishing old furniture. I once built a person a repo and used all square nails.
I once completely renovated a 3k sq ft, 1862, 2-story farm house when I lived in upstate New York. It was built with/by "post, peg & beam" construction and square nails. And since I like to brag about my handi-work, this is what it looks like now.