Proton API, does one exist?
Proton does not currently offer CalDAV or CardDAV services, however there is a highly modular platform used by many large companies that allows for simple integration into third party authentication services: Baikal, backed by sabre/dav.
Does Proton have a published API that could be leveraged for authentication? sabre/dav already has a plugin for IMAP, but of course that can't put targeted to Proton. Many services offer access to their IDP these days, I'm wondering if there is a service like this from Proton as well?
No there's no public available API and AFAIK there are no plans for that.
Thank you for letting me know!
Official API client used by proton: https://github.com/ProtonMail/go-proton-api
I didn't see any good documentation. Its in use by (or has at least inspired) https://github.com/henrybear327/Proton-API-Bridge?tab=readme-ov-file
The API bridge backs rclone's Proton Drive support.
What's caldav and carddav used for?
Usually sync calendar and contacts.
Proto calendar does have an ics thing. Maybe you can use drive to backup your contacts
Not there. But would be great. At least a way to backup and recover your contacts to a new phone (both Android and iOS). That would be awesome.