Intergalactic Digital Research's CP/M was a very close copy of DEC RT-11, though not quite as close a copy as MS-DOS was of CP/M, so CP/M had a great deal to do with DEC.

VMS, which came later and is therefore irrelevant, used . to separate directory names in paths.

Per the article, DEC's TOPS-10.

Can't say how it was back in the day but we use something like [HOMEDIR.SUBDIR2.SUBDIRC] on OpenVMS now.