It will not show any sign at all of being charged when you’ve plugged in the charger.
If your phone is bricked, you will be faced with an unresponsive black screen. And glass, no less! But this is the occupational hazard you need to be prepared for when you are tinkering around with custom ROMs and Recovery images. It can be very disheartening to know that something went wrong when you were flashing files and turned your OnePlus 7 Pro into nothing but a fancy piece of metal.