Linux

Back in the super early era of computers, they were stupidly expensive. One solution was to hook up a lot of people to a single computer via a computer terminal, which were much cheaper.

Basically it would allow you to deploy a ton of monitors and keyboards to access a single computer relatively cheaply, and UNIX was the OS that (mostly) was used for this.

You noticed that your console session is called 'TTY-a-number'. Well, TTY stands for 'teletypewriter' which was the very first incarnation of this, and was what was in use when the name of the console was a made, and it's just... never been changed, though tty devices and their later serial consoles are quite dead as far as tech goes.

Enabling a serial terminal in Linux is a one-line change, and you can then use any terminal emulator you'd want to connect over it, but eh, it's a pretty dead technology and nobody uses that at this point.

Since I seem to be dumping useless retro facts all over the place: you could do this with DOS, and Digital Research released Concurrent DOS to allow multi-tasking, multi-user access to a DOS system. If you wanted to fiddle with that in the modern era, you'd want the Novell Multiuser DOS rebrand, since it supports vt100 emulation and thus can be used with basically any serial terminal app unlike the previous versions which emulated specific HP and IBM serial terminals.