During flashing GPIO 0 is required to be grounded. After flashing, can I use this pin as a output pin as a PWM pin?
I think you can use that pin after flashing.
I haven’t experimented PWM on that pin, so I’m not sure.
Note that pin is connected internally to a pull up resistor. See here: https://i1.wp.com/randomnerdtutorials.com/wp-content/uploads/2020/03/ESP32-CAM-AI-Thinker-schematic-diagram.png