An error occurred while saving the commentJim Bob Howard commented
Agreed. There appears to have been functionality in previous versions what would allow data in a cell to be mapped to a custom property via named range, with said custom property automagically mapping to a SharePoint Library column of the same "name."
Certain references online indicate that this may work best for "Text-to-Single line of text" promotions, with something wonky going on if the SharePoint column is a Date/Time field (probably something to do with formatting). I've not been able to get it to work with Excel 2016 with SharePoint Online from any browser.
In my case, I would like for the SharePoint columns to be read-only, having the Excel data update them as needed. Other options I can see being useful:
1. Excel custom property is read-only, dictated by the SharePoint Column,
2. Two-way sync between them so that either can update the other, or
3. The sync only occurs on instantiation (upload, or creation from "new" menu)