Does USB Power Delivery handle USB cables that are not PD-aware?
USB-PD only uses two wires (GND and VBUS), but the standard has the whole section (4.4 in 'USB_PD_R2_0 V1.0 - 20140807.pdf') which describes how the cable detection is implemented. In short, regular USB-A to USB-B cables need to have the special mechanical mark to be considered for >5V/>1.5A, while micro-usb send special signal on "ID" pin, and determines capabilities from that.

In theory, if you do not use special PD cable, then the devices will silently limit the maximum power to pre-PD levels, and all will be safe (at most 5V, 1.5A current).

In practice, I bet there will be manufacturers which produce very cheap cables which still claim to support maximum power, so we might still read about cheap cables catching fire.