I want to power a 5V relay breakout board from a Pi Zero W. I believe the 5V pads are "raw" and a "pass through" voltage? Is this correct?
On the Pi Zero and Pi Zero W the 5V pins are directly connected to the 5V input from the µUSB connector, as can be seen in the schematics
Other models have additional components (polyfuse, ideal diode etc).
The first Answer tells us that the voltage output from the USB supply is regulated iaw the USB standards, and therefore needs no further regulation on the RPi. So, I'd say the answer to your question is, "the 5V pads are regulated iaw the USB specifications and standards".
The second Answer is not applicable directly to the Pi Zero W, but useful as it explains some details wrt how the USB input voltage is handled on the RPi, and perhaps more relevant is the observation that the published schematics may not be reliable.