Given that Proton is intended for privacy, perhaps a simple and relatively safe default would be to create an XML feed that would only block of busy times. For example, in my use case I would want to view my Proton Calendar, but not any of the details of the meeting in my work Outlook. If I need to have my Proton Calendar show up as "busy" in my work account (so people can see that I am busy when trying to schedule something) then I can use PowerAutomate within the work account alone. Other people might use IFTTT or Zapier. Regardelss, by only showing an appointment I'm not exposing any of my data exposed to other calendars.
David
Given that Proton is intended for privacy, perhaps a simple and relatively safe default would be to create an XML feed that would only block of busy times. For example, in my use case I would want to view my Proton Calendar, but not any of the details of the meeting in my work Outlook. If I need to have my Proton Calendar show up as "busy" in my work account (so people can see that I am busy when trying to schedule something) then I can use PowerAutomate within the work account alone. Other people might use IFTTT or Zapier. Regardelss, by only showing an appointment I'm not exposing any of my data exposed to other calendars.