Tested Shibby and AdvancedTomato version 135:
USB3 LED is never ON, even if a USB3 drive is connected to USB3 port and recognized by the router. But USB2 LED lights up if a USB drive is plugged in into either USB3 or USB2 port. In either case, USB drive operates correctly without data issues.
USB 3/ USB2 LEDs work correctly in Asuswrt and Asuswrt-Merlin.