Historically BRD_PWM_COUNT has been the way to configure which pins are PWM vs GPIO, across ArduPilot. Seems like last year there were some changes to increase flexibility, so more recent firmwares just require setting the relevant SERVOn_FUNCTION to Disabled (-1), or alternatively using the new SERVO_GPIO_MASK parameter 
This is something we’re yet to document because it doesn’t yet affect the stable firmware, but we’re currently in the process of improving our documentation system to support multiple software/firmware versions, so in future these kinds of changes should be documented pretty much as they occur