I just checked, I’m not using the feature but my current ISP still offers it: https://assistance.free.fr/articles/631 (10 GB FTP storage tied to the ISP-specific e-mail address).

Having looked it up, mine makes it an add-on service for 1,045円/month + 5,500円 set-up fee, at which point you might as well use a dedicated VPS service (which is probably what's actually going on behind the scenes anyway).