According to French law, all baguettes must be made of only wheat flour, water, salt and yeast (source).
So, do Jews who keep kosher (and eat pas palter) in France eat baguettes from plain unsupervised boulangeries, and not require a hechsher?
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That is correct and many observant Jews eat this bread (I lived in France for five years). The same is true for many sorts of plain bread, not just baguette.
However one has to check that the oven is only used for plain bread and not for other specialties with cheese or meat, if the supports used in the oven are not covered with grease and if they use the remainder of (possibly non-kosher) pastries from the day before as yeast.
Baguettes are often cooked in special ovens and are therefore most likely to be kosher.
The official kosher product list from the French Consistoire mentions this on page L-32, right side under "Pain Français Courant".