Louis XIII actually outlawed dueling in 1626, and every subsequent king actually swore in his coronation oath not to pardon any duelists. There are a few reasons for the strong outlawing of the practice, one of which was simply that aristocratic deaths in duels deprived the Kingdom of France of military officers (among whom the practice was most common).

Nonetheless, though the practice declined somewhat under Louis XV, it saw a sort of romanticised resurgence under his grandson Louis XVI; in 1778 the Duke de Bourbon, a prince of the blood, engaged the Comte d'Artois, the king's youngest brother, in a duel over some disagreement over how Artois had addressed Bourbon's wife. Neither of them were killed, and Louis XVI accepted a simple apology from his brother, which hardly seemed to coincide with his oath in his coronation to never pardon any duelist.

To summarise, the practice was illegal but it certainly was not dead. Even magistrates were known to flout the rules.