Maybe I’m missing something but was it not possible to apply for permanent residence in France after the U.K. referendum result? Was it only possible before the result? Was it ever possible?
As It Minion explained, it was complex and problematic.
Before and immediately after the Brexit result, you could, as an EU citizen, apply on line, or in person.
But as the agreement did not specify how, who or when, or how long the requirements would be, the system was suspended, and no decisions were made, but any applications were held in abeyance.
The same would have applied to any personal visits to offices, except they would have been simply turned away.
Once the final Agreement was reached, and about October 2020 the system was reopened, and any held applications were processed.
Prior to the Brexit referendum, any application was simply superfluous and not required, and usually not processed by the French.
Any other way, was applying as if rom a third country and meeting all the required conditions.