A response to Michal Necasek’s “Why Does Windows Really Use Backslash as Path Separator?”, also posted on the orange website.
This is almost correct, but MS-DOS derives from the DEC small systems line (the PDP-8 and PDP-11), not the large systems line that ran TENEX, which took the name "TOPS-10" in the DEC Witness Protection Program (the PDP-6 and PDP-10). TENEX's only real descendant in modern computing systems is the command-line editing (and filename completion?) in bash and zsh.
Much to my surprise, it seems to be true that CP/M did not actually use / for flags, even in PIP, although other incarnations of PIP (like that in Heath's HDOS) did use /. The CP/M 2.2 manual is at http://www.cpm.z80.de/manuals/cpm22-m.pdf and documents the command lines of all the standard utilities, including the assembler, PIP, and the text editor. (It also, in passing, documents the full 8080 instruction set and OS API.)
TENEX was born in 1969 but grew up in the 1970s, but the use of / for switches in DEC-land predates it; https://en.wikipedia.org/wiki/Concise_Command_Language is somewhat confused, but it currently describes how the PDP-6 monitor program used / for switches in, presumably, 1964.
DEC operating systems like the ones CP/M aped used / liberally for switches, and third-party programs we used on CP/M certainly did use / for switches. This was not limited to the PDP-10 large systems operating systems; it was also true on OS/8 for the PDP-8, as described in https://www.pdp8.net/os/os8/index.shtml (though, as you can see, some commands used - instead, like the later Unix). The PDP-8 shipped in 1965, but OS/8 might be more recent than that.