What are the unused pins in an Arduino L293D motor shield?
Check out this Link. Here, you can find this explanation in Code. I hope it will helpful for you.

Pin usage with the Motorshield
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Analog pins: not used at all...
A0 ... A5 are still available...
They all can also be used as digital pins.
Also, I2C (A4 = SDA and A5 = SCL) can be used.
These pins have a breadboard area on the shield.
Digital pins: used: 3,4,5,6,7,8,9,10,11,12
Pin 9 and 10 are only used for the servo motors.
Already in use: 0 (RX) and 1 (TX).
Unused: 2,13
Pin 2 has an soldering hole on the board, easy to connect a wire.
Pin 13 is also connected to the system led.
I2C is possible, but SPI is not possible since those pins are used.