I know that Hearts of Iron IV is just a video game.
In almost every playthrough I've had, the victorious Allies release France and an independent Occitania from the clutches of the Third Reich. The only playthrough where this didn't happen was one where the Third Reich won.
Is the game reflecting that the Allies seriously considered granting independence to Occitania?
I'm not familiar with that game, but from what I gather it creates a number of "fantasy" countries based on more or less historical ones.
"Occitania" is one of those: there are many historical definitions of it, some purely cultural/linguistical (which remains the most effective one: Occitanie is the Pays d'Oc where people used to speak one of the "Oc" languages) and other administrative (from the old Languedoc to the current Occitanie region created in 2016). The concept of Occitania is old, but there has been for centuries no long-standing, non-ambiguous political entity called Occitania that could be cleanly separated from the rest of the country.
It may be that the game developers were inspired by one the following:
The French "free zone" or "non-occupied zone" during WW2 was the southern part of France, governed by Vichy but not occupied by the German army. It more or less matched the Pays d'Oc, except from the west coast region. It's the closest thing that we have for a separate Occitania with a "border", but it lasted from June 1940 to November 1942, at which point it was invaded by the German and Italian armies and known as the "Zone Sud".
The Felibrige literary movement of the mid-1800s aimed at defending and promoting the langue d'Oc and its literature. The movement was popular but it was courted by Vichy during WW2 and some leaders became Vichyte collaborators, tainting the whole thing. It still exists today as a cultural association.
The Occitan regionalist movement emerged in the late 1960s in the wake of other similar movements (Britanny, Corsica). Activists, rather than historians, started rewriting the history of Occitania as that of a colonized country (with the suppression of the much-debated today "Cathar heresy" as a turning point). Still, the movement remained largely cultural rather than political and its activists did not call for separatism.
And that's it. Occitania in the 19-20th century was mostly a cultural construct, and briefly an administrative zone during WW2.