Thank you, the article you linked ( https://www.moneyland.ch/en/inflation-price-increases-salt-2023 ) explains all the details and answers all my questions.
Additionally I've found a confirmation in the contract with Salt:
Salt may modify its Services and/or any parts of the Contract and/o prices, charges, fees at any time. You will be informed in an appropriate way, e.g. b SMS, letter, e-mail, note on the invoice. If you do not agree to essential changes to the Contract that are to your significant disadvantage, you are entitled to terminate the Contract in writing within 30 days of receiving the notification
I didn't decide yet if I'll want to terminate my contract. First I wanted to know if I'm allowed to do so without paying any penalties and within what time frame (now I know - 30 days as per contract), because these informations were not included in communication from Salt.