The regions.csv file contains mainly links between old (since 1970-01-09) and new regions (as of 2016-01-01).
The initial date has been chosen given the latest modification (fusion of Corsica and Provence-Côte d'Azur) prior to the 2016-01-01 fusion:
Le décret no 70-18 du 9 janvier 1970 porte à 22 (vingt-deux) le nombre des régions métropolitaines en séparant la Corse de la Provence-Côte d'Azur. — https://fr.wikipedia.org/wiki/R%C3%A9gion_fran%C3%A7aise#R.C3.A9gions.2C_.C3.A9tablissements_publics
id: This is a GeoID.insee_code: The 2-digits code delivered by INSEE.start_datetime: The effective start date + time for the currentidusing ISO format (YYYY-MM-DD HH:MM:SS).end_datetime: The effective end date + time for the currentidusing ISO format (YYYY-MM-DD HH:MM:SS).name: The name of the region.successors: List ofids separated by semicolons which are successors of the currentid. Default is an empty string.ancestors: List ofids separated by semicolons which are ancestors of the currentid. Default is an empty string.population: The population as of 2013.surface: The area in km2.chef_lieu: List ofids separated by semicolons of the Chef-lieux for that region, as found in communes.csv. Default is an empty string.nuts_code: The 4-chars code delivered by Eurostat.wikipedia: The wikipedia page for that region (note that given how recent the change is, some links still redirect to temporary region names).
Given that regions have been merged, there is one line by current region and one line by ancestor. The id and insee_code columns are unique, the nuts_code one is NOT.
Regarding dates, the initial date + time has been set as 1970-01-09 00:00:00 given the date of the related décret n° 70-18. Arbitrarily, the far future end date has been set to 9999-12-31 23:59:59.
This is a regular CSV file with value separated by commas and a header line with previously described column names.
You can have a look at the file using Github preview if you click on it.