The trend towards a beveled 3D look in GUIs only really started after NeXT started showing NEXTSTEP in 1988. It was a big differentiator for the NeXT interface; it made people sit up and take notice.
Before then by and large GUIs had a flat look. The only depth was a bit of "cartoonish" drop-shadow on windows and menus on the Lisa and Mac (much more subtle - 1px - on the Mac) and on the minimize/maximize/close button contents on Windows 1 through 2. (And of course Amiga, Atari ST/GEM, GEOS, Xt and SunView all mimicked the Mac to various degrees.)