Back.
Nowadays, you should be able to find "proper" French restaurants serving vegetarian main courses (just one, mind you 😆) in most reasonably sized towns, especially those with an alternative/ student population.
And why wouldn't you?? Even in France a decent chef has a sense of adventure and likes to innovate and incorporate different influences into food- and doesn't France produce great vegetables, pulses and dairy products?! IMO it's the mark of a timid, unskilled chef if they need meat or fish to create a good dish- and vegetarian dishes must have better profit margins, too.
Even Buffalo Grill (those roadside restaurants with the horns!) have a veggie burger option nowadays, and they don't even make you sit in the car park to eat it 😆
In November I had a fantastic meal at a restaurant called Lautrec somewhere in the vicinity of Pigalle in Paris- delicious oyster mushroom tacos. Paris is fairly easy, obviously- as well as the aforementioned falafel and other ethnic restaurants, there is a whole row of South Indian vegetarian restaurants on Rue Fauborg du S.Antoine (I think it is) down the side of the Gare du Nord- very handy if you're travelling by Eurostar.
I also found Marseille a real foodie heaven, although I don't think I had any traditionally French food while I was there last year- just Spanish, North African and Japanese- and it was all delicious.
Don't forget the option of looking up vegetarian restaurants- or just looking at menus- on Google maps, rather than trudging all over town.
Out and about, boulangeries often have some delicious quiches without meat, even if they usually refuse to make you up a fresh cheese and ham baguette without the ham (whyyyy? 🙄). I may have caved and had a smoked salmon baguette while with the kids in Paris last week (I was not going to drag them all over looking for suitable options just for me, fuck it).
In terms of self catering, supermarkets are even starting to stock rather dodgy looking frozen meat alternatives, and if I've had Carrefour's spinach and goat's cheese quiche once I've had it a dozen times.