What is this component C21SC?
I have a similar component excepting the package, which is DIP-8.

Indeed it is 24C21 (mine is 24LC21 and marked as LC21) and it is an I2C memory designed for computer monitors, to store the EDID.

It has 1 kbyte.

It is direcly connected to pins 12 and 15 of VGA connector and may have a read-only or transmit mode, because its content shouldn't change.

Even though it is designed for monitors, you can use it in any circuit, as long as you connect it via I2C bus.