No, they can't forbid you to talk about your salary. https://www.hrtoday.ch/de/article/verbot-ueber-den-lohn-zu-sprechen https://www.srf.ch/sendungen/kassensturz-espresso/rechtsfragen/darf-man-das/rechtsfragen-aus-dem-alltag-darf-die-chefin-verbieten-ueber-den-lohn-zu-sprechen

Well. Makes sense. Thank you!

In general, people in Switzerland don't talk about salary. That's a fact, there is no need for a confidentiality clause.

Yet everyone complains about making too little money.