Is money earned from working in a restaurant that sells expired food items considered illicit (haram), and what should the worker do?

Your continuation of work after knowing that expired materials are being used is forbidden, because it is cooperation in sin and aggression. Your repentance, leaving the work, and advising its owner is a good thing. As for the money earned, there is a forbidden part corresponding to the cooperation in the forbidden act. If you have spent the money, you are only required to repent. If you have saved any of it, then estimate the part corresponding to the provision of corrupt materials and assistance in deception, and donate it as charity. If you are poor and in need, it is permissible for you to take from it according to your need, and then donate the rest as charity.