Like all Creoles, they were indeed granted citizenship (likely with varying degrees of reluctance) and eventually absorbed into the American community.

Haute-Louisiane never had the population density of Basse-Louisiane and was therefore anglicized more quickly and more easily than its counterpart to the south. However, French did persist in certain pockets into the twentieth century. Alongside Louisiana French and New England French, the oft-unmentioned third variant of native North American French is Illinois Country French, which is today spoken by a very few elderly (as in, perhaps several dozen) people in Old Mines, Missouri. There are several activists in Missouri campaigning for the preservation and continuation of this dialect, sometimes drawing support from the much larger and more active Louisiana French activist community.

Creoles in urban centers like St. Louis, similarly to its southern cousin New Orleans, tended to assimilate more quickly than did their cousins in the isolated countryside. (Kate Chopin, among the most famous Louisiana Creoles, was herself not born in the modern state of Louisiana; she came from a family of St. Louis Creoles.) St. Louis, which was bolstered by waves of immigration from the still-francophone areas of the pays des Illinois, did maintain several francophone newspapers throughout the nineteenth century, but they did not survive to the twentieth century and we can assume that French had become uncommon by that point. (By contrast, New Orleans' last francophone newspaper, L'Abeille de la Nouvelle-Orléans, persisted until the late 1920s.)

By the mid-1900s, linguists were commenting on the rarity of French even in the most isolated communities, estimating that the number of Missouri francophones numbered perhaps several thousand. (To provide a point of contrast, data from 1970 indicates the presence of nearly one million francophones in modern-day Louisiana, and the 2020 census data indicates nearly 100,000 speakers of Louisiana French—itself probably an underestimate because the census misses francophones who do not speak French in the home, which includes many of the francophone youth and most francophones married to non-francophones.)

So while there is not an exact answer to your query because the Midwest is not a monolithic entity, French has been very much a minority language since the turn of the twentieth century and has been in more or less steady decline since the transfer of Louisiana in 1804.