https://www.cestlagreve.fr/secteur-d...ite/transport/
In the 48 hours or so prior to general national action you can get a bit of a heads up on the effect on aviation by looking at:
https://www.public.nm.eurocontrol.in...pec/index.html
and checking to see he if anything is flagged up such as "ATC Industrial Action in France".
The reality has been that there has been the very very odd complete shutdown of the system, but on most days of national action, even the biggies, many flights have continued to operate but DGAC (the French version of the CAA) reduce flow through nominated airports, typically by 20% - that gets flagged up on the Eurocontrol site.
At our local regional airport that has typically meant for example that on most strike days the likes of BA and Easy will go down from three flights a day to two, rather than the airport being completely closed.
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Or are French ATC employees just calling in sick in large numbers, doing a go-slow or work-to-rule on days when they are not officially on strike ?"
Not from what I have seen, outside of strike days it appears to have been ops normal as far as ATC go.