I have just checked & you will find info on CSS at
www.dgac.fr then "espace professionel" then "examens du personnel navigant" then "CSS"
Good luck
Yes AF did go through phases of hiring without it but that did not mean you did not have to pass it ! You were hired, paid minimum wage until you passed the CSS and you had max 3 attempts, if not, OUT...
Now they have stopped those "contrats de qualification" & you need the CSS to be able to apply to AF...