I spent 3 months cycling all over france back in 2013, and long trips in subsequent years - pretty much every campsite I stayed at used captive portals. A handful handed out password cards to guests only, but those ones were almost always <place name>2013. Back then I didn't have much choice as mobile phone reception in the Alps was non-existent. The campsite owners were aware of the law, they'd often mention that they weren't happy about it.

Last year I was in Paris, but didn't have to fight the system any more, 4G reception was fine. So, no idea what it was like in cities, but in the countryside they followed the law.