The idea sprang from the French government, which wanted a payback after bailing out Air France during the pandemic. It’s not a total ban, but means any destination within the country that can be reached by rail in two-and-a-half hours cannot by operated by an airline on environmental grounds.
That broadly translates to eight routes so far, although it’s a confusing picture. Services between Paris Orly and Bordeaux, Nantes and Lyon are prohibited because Orly is classed as a point-to-point airport and because journey times are short.