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You don't even need netcat or socat for that, probing /dev/tcp/<host>/<port> from the shell is enough.

Telnet was available in the 90s. I reckon /dev/tcp is way more recent. GP did say a long time ago.

That's some gnu bash shenanigans. There is no /dev/tcp in unix

Lots of shops didn't have gnu installed: telnet was what we had.