> Flexbox is a layout system for 1D content.

An individual flexbox container is one dimensional. To get a 2D layout, you nest rows and columns.

> ...doing a responsive layout in explicitly 2D terms is a very weird thing to be doing and usually isn't what you actually want.

I have no idea how you would come to that conclusion. To me, a "responsive" layout is one that adapts to the viewport size. The viewport is 2D. This is a 2D layouting problem.

A subset of that problem is concerned with fitting content of a fixed viewport width to a variable height, i.e. your classic web site. That's what you may call "1D content". Once you're trying to get a layout that adapts to both viewport width and height (i.e. an ordinary application), CSS can get quite ugly.