I'm reading that the esp-idf 5.0 branch switches to GCC 11 with proper C++20 support. But there is no corresponding arduino-esp32 branch yet. Does this mean I cannot use the new GCC 11-based platform with PlatformIO? Or is there a way?

You can inspect the schematic, there is no 5V level shifting for the GPIO. The only thing taking 5V is the usb connector, the CP21 usb to serial, and the LDO. That in itself doesn't mean anything. Atmel SAM3X8E, the chip in the Arduino Due board, is a 3.3V chip and doesn't receive 5 V voltage, nor ...