Use ESP8266 ESP-12F/E D1 Mini GPIO Pin 10

dontdothis.ino

/*
 * In ESP 12 modules, spec. D1 Mini mounted, GPIO9 and GPIO10 can sort of be freed up.
 * GPIO9 is connected to flash HOLD
 * GPIO10 is connected to flash WP
 * 
 * In DIO mode, these pins should be free, but remain connected to the flash chip.
 * Using GPIO9 will crash the module sooner or later, GPIO10 is relatively safe, but
 * don't fuzz it. Might be good enough for SPI CS or similar, no data rates pl0x.
*/

bool enabled = false;

void setup(void) {
  Serial.begin(115200);

  enabled = ESP.getFlashChipSize() == FM_DIO;

  if (!enabled) return;
  
  pinMode(10, OUTPUT);
  digitalWrite(10, LOW); // let's see if this'll brick it
}

void loop() {
  if (!enabled) return;
   
  Serial.println("Setting high.");
  digitalWrite(10, HIGH);
  
  delay(1000);
  
  Serial.println("Setting low.");
  digitalWrite(10, LOW);

  delay(1000);
}