Going through the deleted GPOs and doing "Unregister and Remove history". Some GPOs throw the following error.
The following error occured executing the stored procedure or method DeleteAllBackupsAndUnregister: The DELETE statement conflicted with the REFERENCE constraint "FK_ObjectData_BackupData". The conflict occurred in database "GPOADmin", table "dbo.ObjectData", column 'LastBackupID'., gx_nis_qgpo_p001, Error, 2250, Error,Microsoft.Data.SqlClient.SqlException (0x80131904): The DELETE statement conflicted with the REFERENCE constraint "FK_ObjectData_BackupData". The conflict occurred in database "GPOADmin", table "dbo.ObjectData", column 'LastBackupID'.