The suburbs and smaller towns (Nantes, Chartres, Bordeaux, Toulouse etc.)
I saw a ton of friends and family move out of Paris as they started building a family or wanted more than just the urban life.
It's actually still extremely easy to get to Paris from more than 20~30km away. Many keep commuting to Paris while taking a healthy distance.
I'm surprised, and so my statement above is wrong as more people in Paris are born outside of it, I would have thought it was more than 50%
That said, I wouldn't be surprised if the greater Paris (basically the extent of where the RER goes) had more natives moving in that sphere for most of their lives.
- 1861: 57% born outside of it, 1872: 64%, 1881: 62.9%, 1901: 57.4%.
So the fact that most Parisian has been born outside of it is not exactly new. What probably changed is the part coming from outside of France. But it's not exactly as if Paris was not described as Cosmopolitan since at least a century so not sure how that would be bad.