Windows 95's 3D look & feel was copied from NeXTSTEP. To me the pinnacle of 2D UI design was OS X from Steve Jobs' time. https://en.wikipedia.org/wiki/Mac_OS_X_Tiger#/media/File:Tig...
I miss the brushed metal theme too, but Tiger never looked quite right. There is way too much white space in that finder window, especially compared to the way the dock packs them so close together. The icons on the side bar not matching the folder icons is also annoying, when the document icons already don’t match the application icons on the dock. There are three different symbols on the screen for music, and that doesn’t make visual sense.
Looking at NeXTSTEP screenshots, I would say that what got copied was the grey color palette used to make the widgets non-flat. But the actual widgets are rather different, that aside.
And didn't NeXTSTEP built on Motif in turn? Is Motif the granddaddy of "chiseled 2D", or was there something before it?
IMHO it was the pinnacle of 2D UI design. Clickable areas were visually obvious, and there was no ambiguity about what happened and why.
And now we live in a world where fashion rules over function.