A related question that is also interesting: What is the oldest laptop that can run an up-to-date, internet connected OS (i.e a museum device with the same OS from 1991 doesn't count). It doesn't have to have a fancy desktop, but it does have to support connectivity to the internet, and run all the latest security patches and updates. This is different than the article because optimizing productivity isn't an objecti…
For really, really old hardware, I suppose FreeDOS is always an option-- they're currently working on version 1.3, so it would theoretically count as something that's "up to date" - although I wouldn't actually trust FreeDOS software to be secure on today's internet. It does have an auto-updater of sorts, though, FDIMPLES.