If a requisition is physically deleted by the MRP run because it is no longer needed, then you cannot get it back, it is just gone. If the current stock cannot fulfill new demand, then the next MRP run will create a new requisition.
If it was deleted by an archiving run, then you cannot get it back with existing functionality, as this is not foreseen by SAP. But archived data is not lost data, it is just stored somewhere else, and I know about companies who developed programs to restore data from an archive.
Of course they did not do it for a single PR and they did a deep analysis for some weeks to evaluate if that could create inconsistencies, and the result is just very case specific and cannot be adopted as general approach.