Even more “serious” machines had minimal to no directory structure in many cases! On the DEC side of things, for the PDP-11, RT-11, RSX-11M, and RSTS-E were all flat filesystems or had a single level directory structure with no support for nested directories. I believe many of the PDP-10 operating systems had similar restrictions. I don’t think that it was until the advent of Unix that directory nesting to arbitrary depths became “normal” even on minicomputer disk operating systems.
As you say, though, by the mid-1980s (and certainly by the late 1980s) even micros had nested directory structures on a wide variety of platforms (including GUI environments on the Macintosh and Amiga by the 1985-ish time frame).