I sat down to log into a couple MUDs yesterday. Didn't stick with it

the combat still hasn't evolved enough for me

but you could play that on anything capable of displaying text on a screen and running telnet.

It looks like some crazy person has been doing a TCP/IP stack for the 128K Mac, the first Macintosh ever released, from 1984, as well as a telnet client. So you can technically lug a 42-year-old computer out of an antique store and play currently-being-developed Internet games...though you won't be getting color, since that came a while down the road.

If you get some device that can expose a serial console on some system to TCP/IP

not sure how far back you need to go for that

you could technically play it on a teletype from the 1930s.

The "some device" will have to be later, though, so that's maybe kinda cheaty.

Technically, Debian Linux has been run on an Intel CPU from 1971, but it isn't fast enough to be a practical host for such a teletype in that environment. Even stripped down forms of Linux are going to be "too big" to be such a host.

It does sound like the Commodore 64 has a package, Novaterm 10, that runs TCP/IP and telnet, but I don't know whether it can output to the C64's serial port rather than video display; you could play locally on one of those, but probably not run on a teletype. That being said, it probably shows that it's technically-possible, since I'm sure that if it can run a virtual terminal program, it has the resources to just dump the text to an actual terminal via a serial port. I'd guess that there's probably some system out there circa 1980 that someone has probably built that can both run a TCP/IP stack and expose a serial console to a 1930s teletype to play current Internet games.