Comment by skissane

> all of the old flat file systems with no directories (like on the original MacOS, CP/M, MS-DOS 1) are all comprehensively obsolete and ignored.

not quite true. The classic flat OS/360 filesystem is still heavily used on IBM mainframes. Of course, probably your app will never run on a mainframe, and if it does, it will probably run either under z/Linux or the z/OS UNIX filesystem (zFS). But there is still a lot of actively supported in-production software which does, since born-on-the-mainframe apps mostly use the classic filesystem and the POSIX file API is mainly used by apps ported from elsewhere