What is the "sep=" metadata you can add to CSVs?

Solution 1:

RFC 4180 is commonly recognized as the standard for the CSV format, and does not mention any such feature.

W3C Model for Tabular Data and Metadata on the Web mentions it as a feature not within their scope:

Many "CSV" files embed metadata, for example in lines before the header row of the CSV document. This specification does not define any formats for embedding metadata within CSV files, aside from the names of columns in the header row.

So while I did not find any direct proof that this feature is Excel-specific, this apparently still is the case as there seems to be no standardization document covering that feature.