Did people ever actually observe "no white after Labor Day"?

In my Boston childhood, definitely. White was only worn between Memorial Day & Labor Day.

Heck, I’ve been to a wedding where the bride wore black. (No, she wasn’t a goth or anything. It was a classy, hip, outdoor wedding, and it was just a pretty dress she liked.)

Ditto here - Chicago area. In the 70’s, early 80’s.

The only differences between the picture of my Greatgrandma’s First Communion and her wedding are the color of the mantilla (white for FC, black for wedding) and the presence of a groom in the second one. The dress is the same black one. I’ve got part of the sew-on-sew-off decorations.

The one time I attended a wedding in the US, I wore a black bolero and a large, heavy, black skirt, with a white blouse. It happened to be my best clothes. Thankfully nobody took offense (Cuban-American bride, Armenian groom; MotG also wore black which could no way be mistaken for mourning clothing).