I would say the 1D/2D thing is a summary/rule of thumb. But grid can be useful for "1D" (linear) layouts too -- I feel like I've used it for something that was

I would say the 1D/2D thing is a summary/rule of thumb. But grid can be useful for "1D" (linear) layouts too -- I feel like I've used it for something that was harder to do with flex even though it was linear, although I can't recall specifically. And flex can certainly be useful for some "2D" things depending on what you mean by that. Grid is newer, and somewhat more complicated. Newer is probably the main reason it seems less popular than flex.