French operator SNCF has previously asked passengers to self-declare as ‘Monsieur’ or ‘Madame’.
The EU’s top court ruled on Thursday that requiring rail passengers to declare a gender when buying a ticket is in breach of the EU’s General Data Protection Regulation (GDPR).
Yeah mostly, “prefer not to say” kinda means it’s optional. You’re allowed to use optional non-essential stuff like this, you do have to state why and not use it for other things. So they could use “Madame Lastname” to address their emails to you if they say that’s what it’s for but not reuse that for market research.
I put it like this because that would have avoided the complaint and most likely the court case too.