RTS Pin on telemetry port does nothing

[gezzuzz]

Issue details

RTS Pin on telemetry port does nothing. I set BRD_SER1_RTSCTS to enable. CTS works fine. No transmit is made unless the CTS is low. But RTS always remains low even when there is no data to send. I was expecting (or would like it to be) When there is no data to send that RTS is LOW and HIGH when there is data.

I started testing with Pixhawk 4 mini but also tested Pixhawk 1. I get the same results.
Tested with ardusub and confirmed the same thing with ardupilot

Version
4.0.0

Platform
[X] Plane
[X] Submarine

Airframe type
SUB

Hardware type
Pixhawk 1 and Pixhawk 4 mini