In the ProtonMail Threat Model document[1], in the "What ProtonMail does not guard against" section, you should add "Dissenting Opinions".
I know of at least one case where a vocal right-wing personality had his account removed because he listed it as his contact.
I understand ProtonMail is a private company and can do what they want. So they should be honest about not being a safe place for dissenting opinions.