One method for deleting specific CIs not discovered in 30 days is to use a scheduled job that runs a script to query the CMDB for CIs that have not been updated by Discovery for a certain period of time, and then delete them or mark them as retired. This method can help you keep your CMDB clean and accurate, and avoid having stale or obsolete CIs. You can configure the scheduled job to run at a specific frequency and time, and specify the criteria for selecting the CIs to delete.
References: Automated way to disable/retire CIs not updated for 45 days, ServiceNow CIS-Discovery Sample Questions