Originally Posted by smashndash

This is colloquially known as a “dork disc”. Don’t know what the official term is. If your derailleur is improperly adjusted or gets knocked during a fall, this disc protects the derailleur from getting caught in the spokes. Saves you money and possibly skin.

This one is kind of small, so it might just be there to keep the chain from jamming itself between the cassette and spokes.

I’d recommend keeping it.

EDIT: Too slow

”Properly adjusted” entails setting the “limit screws”, which will prevent you from overshifting. You’ll feel a hard stop if you try.