Openpyxl does not read/write worksheet custom properties
These Worksheet.CustomProperties are not to be confused with Workbook.CustomDocumentProperties.
- Worksheet.CustomProperties: https://docs.microsoft.com/en-us/office/vba/api/excel.worksheet.customproperties
- Workbook.CustomDocumentProperties: https://docs.microsoft.com/en-us/office/vba/api/excel.workbook.customdocumentproperties
They are similar, in that it allows users to save data outside of a sheet/workbook in a collection of objects, each with a name and value.
Workbook.CustomDocumentProperties are saved to the Workbook and have been implemented (pending merge) as part of issue #1003 (closed) (and Worksheet.CustomProperties were also discussed and I expressed that I would like to add this feature too)
Worksheet.CustomProperties are slightly more complicated, as there are not only .XML files involved. Each CustomerProperty is stored as a .bin file, which get's referenced in the "xl/worksheets/sheet{n}.xml" and the "xl/worksheets/_rels/sheet{n}.xml.rels" and the manifest "[Content_Types].xml"