The power LED on the Pi 3B+ is connected across 5V and 0V so lights up whenever power is supplied, whether through the micro-USB connector, GPIO or test points. The LED can however be shorted out and turned off by using the STATUS_LED_R control line.
Unfortunately the full Pi 3B+ circuit diagrams are not available and the reduced schematic does not show where the STATUS_LED_R signal originates from.
Only someone from the RPT engineering team or with access to the full circuit diagrams or internal Pi information will be able to help further.
Or someone who has the skills and patience to reverse engineer or figure things out anyway ...
"The PWD_LED, it is still connected to the GPIO expander that is only available using the mailbox interface.
We posted some info last year about the GPIO expander, that interface still works on the Pi 3B+ except the red LED has moved to pin 2 and is now active low instead."