A few months after leaving the company, the company sent a text message saying that my resignation took effect and asked me to go back and handle the handover procedures.

Need to be dealt with. If the company does not handle the resignation procedures for employees, there will be cases where the company does not give employees the resignation certificate, and employees' entry into other units may be affected. If the employee applies for resignation and meets the time limit, and the company does not handle it, then the company is illegal, and the employee can complain to the labor department and bring trouble to the company.