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 more practical use I'd draw the line to 2003; with T40/R40 you got USB 2.0 and Wifi, both things that make life much easier. From spec point of view you got Pentium M CPU (with SSE2 and all) and top end models shipped with 512M of RAM (upgradeable to at least 1GB!).
That also serves to demonstrate what dramatic progress happened in just 3-4 years.
The next 3-4 years also saw fairly significant improvements, T61 from 2007 has SATA (allowing SSD upgrade) and first 64 bit CPUs, supporting up to 8 GB of RAM. That gets you in the realm of mostly practical daily driver.