Sure it was. That's why it allows multiple user accounts with a number of privilege levels, and gives each user a separate directory tree for storing documents etc.

OS9 and below were not.

However, OS X and macOS definitely were.

I don't blame MacOS- there was no expectation that an OS for 16-bit CPUs with no VM, no protected memory, and only cooperative multitasking would have proper multiuser support. With that kind of hardware, what's the point? Good thing macOS inherited all that stuff from BSD and Mach.

I mean, it's Unix, so you could say it was designed for multiple users on one machine?

Weird, here it reads

$ wc -l /etc/passwd

     108 /etc/passwd