The single feature that is preventing me to move fully to Proton Calendar is this one. Many apps (airlines, medical apps, etc) allow you to "add" the event (flight, medical appointment) to your calendar and of course, since Proton Calendar does not have a "calendar" account in the mobile operating system, this doesn't work. Events cannot be added directly to Proton Calendar in one click.

So I also need to have an Apple Calendar for this type of things, suscribe to it in the Proton Calendar app and then it will show. However, travel information and medical appoinments are things I would like to keep private in a calendar such as Proton.

I think that Proton could integrate something like this - an ics "temporary" calendar that can be integrated into the OS, the event gets added there, this gets uploaded to the Proton servers, encrypted and downloaded into your app and deleted from the "temporary" ics calendar.

Not as secure because the information from your device to the Proton servers would not be encrypted, but anyway it would be deleted really fast and the you'll have everything into Proton calendar.

Not sure if something like this is possible.