Maybe it's just my own perception but it is one of the more widespread "out there" belief in France. It's even supported by national agencies, see my other comment.
Yeah, but these people typically target antennas, not 3 years old phones. And don't work for the ANFR. Source: french, living in a small town with its own anti-EM association.
I don't know the spread of this movement but it does exist. For example, the national electric company deployed a new generation of electric meter (Linky) which can communicate data to your supplier using PLC. This communication of course emits EM waves so some people objected to the installation of this meter.
IIRC the EM thing was marginal, a bigger deal was privacy (behaviours could be inferred and resold), and most of the outrage was that people feared being charged more.
It even predates 5G by a long shot, I recall anecdotes running the news headlines about some folks claiming newly installed (2G? 3G?) cell towers were causing them debilitating headaches, and covering walls and windows with tin foil made it better. It was later revealed that while the tower was indeed up it was entirely unpowered the whole time but that bit of news of course did not make it to the headlines.
But ironically those people were truly and seriously getting headaches and tin foil really made it better. People get to experience what they believe deep in their subconsious. See the Nocebo effect: https://en.m.wikipedia.org/wiki/Nocebo
Having 2 doctor parents and some amount of experience in the field of hypnosis, I can say these sort of things happen all the time to people.
Yes I don't think people realize how widespread stuff like homeopathy, cellular EM scare, antivaxx are widespread in Europe and the rest of the world. The US gets the spotlight but in other countries those opinions are often not even controversial (especially for less obvious stuff like radio waves etc)