Then something more: I've learned that best is that a file contains - in a way or another - the version of the protocol/documentation, to allow in the future, if anything is changed/added, still handle everything correctly or know from start it's not a supported version. Of course, then the use-in-excel may no longer work.
This is generally good advice and has been suggested elsewhere. However, the 'use-in-excel' goal makes this tricky. If such a version must be specified but is not present (and all other data is fine), should the import fail? In general users won't know which version number to use, even if it is readily possible to add it (say in the column headers). Also, it's difficult to see how this format could be extended in a way that required a version number to differentiate, so again I'm not sure it's worth the cost.
Indeed, if use-in-excel is so important, this can become tricky (there can be a column with the version, but it's imho more a hack than a proper solution).
If the version is missing - again, it depends.. one direction would be that the "version info" could also contain something (a name) that will ensure one really imports what he expects, not just a random csv.
Another direction could be to assume it's
at least version 1 (if version info is missing). Of course, this kind of approach may just "pass the responsibility" for finding a solution to those proposing an extension (or a new version).