Dueling was never legal in France, although there had been some legal changes in the previous century which made it slightly less illegal if you can appreciate the difference. Namely, specific laws which named dueling as a crime in and of itself were removed from the books in the beginning of the 19th century, meaning that the constituent crimes which made up dueling were now the only legal offense. In practice, this meant authorities generally didn't take notice of a duel except in the case of death - which was still legally murder - and as dueling in France, by the middle of the century, had developed into a relatively harmless endeavor characterized far more by posturing and signaling of masculine virtue, death was almost unheard of.
Dueling in France reached its apogee during the period of the Third Republic, in the wake of defeat in the Franco-Prussian War, where it became particularly resonant and gained something of a republican character, practiced heavily by politicians, journalists, and members of the bourgeoisie [some more on this here). The appeal of the duel lasted essentially right up until 1914, with only fluctuations in its popularity but never anything close to a complete rejection, but the First World War presented a fairly massive break. This was in the literal sense, in that duels were mostly put on hold during the conflict, with interpersonal quarrels recognized as less important than France's existential fight for existence against the country that had humiliated her a generation prior, but it was also in the figurative sense as well. When the war finished, with millions of men killed or wounded, and millions more returned home having served honorably, the duel lost a certain luster.
It is a pattern that plays out not only in France either, as similar steep decline in the duel in America happened in the wake of the Civil War, and both Italy and Germany experienced similar sharp declines after the end of the Great War as well. Likewise in South America, which avoided the war, the duel saw a slower decline despite similar pre-war practice. Dueling has always been, in large part, about proving ones manhood, and in all cases, the generation that had seen war and proved themselves against the guns of the enemy simply felt they had less need to prove themselves by meeting on the dueling ground. An insult no longer needed to be wiped clean by exchanging shots or lunges to show you were a man when ones war record merely needed to be gestured at. And while the previous generation had many veterans from 1870-1871, not only had they lost, a defeat that resonated for the next forty years in the ideals of French manhood, but the sheer scale of death and destruction of the First World War paled in comparison to anything prior, helping to enforce the new paradigm.
But, while the war and its chaos may have placed a massive break with the past in the French psyche, that didn't mean that dueling was completely gone, rather that it went into a fairly quick, sharp decline. The occasional duel continued to be fought during the interwar period, but it had quickly transformed into an oddity, a relic of the past, and no longer carried the socio-cultural cache that it had prior to the war. The Second World War created another cultural break-point, as well only reinforcing for those few duelists just how strange it was to engage in the practice, which at that point can only be viewed as little more than a publicity stunt, with many years separating the few duels that happened, while a half-century prior duels happened in the hundreds every year. And given the curosity, and their implied purpose, a number of those late duels were documented on film even.
The interesting thing about the 1967 encounter is that when Gaston Deferre and Rene Ribiere had their argument and decided to settle it with épées, it was the first duel in France in almost a decade, the last one prior having been a 1958 duel between Serge Lifar and the Marquis de Cuevas. It was caused of an argument about the ballet, and it too was filmed. In the interim though, Robert Baldick had published his The Duel: A History of Duelling, the first notable modern work to treat the broad topic of the title (as compared to the 19th c. works of Milligen, Sabine, Steinmetz, or Truman) in 1965, which meant that he had already declared the Lifar-Cuevas encounter to be the last notable duel in Europe, unaware that there was still one more to come!
In any case though, the 1967 duel sits there at the very tail end of the post-WWI decline, as duels became few and far between, and France would go for years without any clashes of the blade. It is questionable whether, given the extreme spacing between those last few encounters whether it even, properly, ought to be considered as part of that earlier tradition or conscious pantomime of it instead, although certainly in both cases duels were done for public consumption.
Further Reading