Here in the Pedestal Docs it talks about the following steps in the pedestal dataflow:
Here in a significant project by one of the authors of Pedestal - we see the following elements
My question is - is derive an undocumented feature of the dataflow? Is it the new name for combine? Is this simply a matter of the Pedestal docs being out of date?
As far as I know, yes, it's just a matter of the docs being out of date. The pedestal authors updated the dataflow quite recently from version 1 to version 2.
Most of the existing documentation refers to version 1 (which has the steps you mention), and last time I checked, the documentation for version 2 was still in progress.
However, there is a comprehensive pedestal tutorial available now, which uses version 2 of the dataflow (including derive).
In addition to this, Taylor Sando did a useful write up for beginners