Well over the past couple of years, the RATP (the local authority that runs the Paris transit system, including metro) had been reducing the number of stations in which you could still buy paper carnets of tickets (a carnet being a pack of 10 tickets).
As of Thursday, September 21, they have finally completely ceased selling paper carnets of tickets. There is no longer any window or machine at which you can buy a paper carnet.
***You can still buy individual paper tickets at the machines.
You can still buy an electronic carnet by purchasing a Navigo Easy card (for 2 euros) and then loading a “carnet” of 10 tickets onto it (they still a “pack” of 10 tickets at a price advantage to buying 10 tickets individually). (You can buy a carnet, i.e. 10 tickets for your Navigo Easy card for 16.90 euros. Individual tickets are 2.10 euros each.)
However, what is lost is the advantage that one had with a paper carnet of tickets: you could keep some tickets for yourself, give some to your traveling partner(s), etc. With the Navigo Easy, each traveler must have their own individual card with tickets on it — you can’t share a card.
Here’s an article (in French) with the info: