I've been experimenting with syncing iCloud to EDS. It's mostly possible, but it requires some additional steps from the user.

Users will need to create an app password here: https://appleid.apple.com/account/home

There is an apple support article for this here: https://support.apple.com/en-us/HT204397

It looks like the app passwords are in the format: xxxx-xxxx-xxxx-xxxx if we want to enforce/validate that like we do with credit card numbers in AppCenter

Evolution seemed to be able to fetch a new subdomain automatically when supplied with https://caldav.icloud.com. For example this becomes https://p23-caldav.icloud.com/12345678/calendars

Similarly contacts.icloud.com works fine for carddav

From here, Evolution has you select one single calendar or one single task list or one single contact list each time. I can imagine we probably want to just grab all the calendars, tasks, whatever we can get.

It seems like at least Calendar and Tasks is regular caldav, but I can't confirm that Notes uses CalDav. It's possible that Tasks is also moving to a proprietary format so arg. At least we have calendars and contacts?

For iCloud mail, Apple has a convenient support article here with all the required info: https://support.apple.com/en-us/HT202304

IMAP info:

Server name: imap.mail.me.com
SSL Required: Yes
Port: 993

SMTP info:

Server name: smtp.mail.me.com
SSL Required: Yes
Port: 587
SMTP Authentication Required: Yes

And that's it! So pretty much we direct users to make an app-specific password and then we get iCloud Calendars, Contacts, Mail, and Tasks (for now)