If we need to have a report of all the items (Folders, documents) shared externally in a SharePoint Online (SPO) site, there is a new option to generate such report available in the Site usage page. You only need to browse that pge, and you will find this new option under the “Shared with external users” section:
When you click on “Generate report”, we need first to choose the folder where the report is going to be saved or create a new one. As you can see, you can only save the report in the default document library created when the site was provisioned:
Once you click on the “Save” button, the CSV will be generated in the location chosen. That CSV will have a row for every shared item in the site and each row can have the information described in the following support article:
As a final note, this Sharing report feature also send an e-mail notification to the user when the report has been generated. Bear in mind that if you have required metadata in the document library where the report is going to be saved, the report is not going to be generated. This is a bug I have already reported to the SharePoint Team.