I can confirm, that these pins are working on my C3-MINI-1, as you wrote: I2C(4, 5) and SPI(2,7,6)

Thank you for your answer.
Is it set by Pull-Up or by logic value? Can I connect Pull-Down resistor directly on this pin?
If it is only pull-up, I can at least lower it by some external resistor and then set it to low.

Hello, I can't find answer for my problem, so I'm starting my first topic here. I have got custom made board with ESP32-C3-Mini, where is controll of the external motor connected to UART TX pin (GPIO 21) :( . I'm able to reconfigure the pin after bootloader start (with bootloader_before_init() hook)...