SAP PR can be deleted after PO is Issued
SAP Materials Management
Purchase requisition is used as a requirement document in procure to pay setup. Release levels and approvals are generally obtained in the PR document , based on which RFQ / Quotation or Purchase order is issued. If Purchase orders are issued with reference to purchase requisition , business controls warrant that PR document should not be modified after converting to PO , especially deletion of PR line item after converting to PO.
But in standard SAP configuration , PR line item can be deleted even after PO is issued. A warning message is issued when PR line item is deleted which has a PO linkage but would not restrict the deletion process.
If the business requirement is to restrict the deletion of PR line items then the easiest way to achieve it is to make the warning message issued during deletion to error message by a simple customization. In case if the requirement has validation and checks before deletion then PR user exit can be used.
How to restrict PR line item deletion suing customization
Navigate to SAP Customization menu : SPRO --> Materials Management --> Environment Data --> Define Attributes of System Messages
Reference
521174 - ME52N: Deletion of a PReq despite PO