Uh, I don't think IMAP is generally blocked on iOS? Anyway, end-to-end encryption seems to depend on the email provider being able to support IMAP. So Hey.com is out but gmail can work if you enable IMAP support for example.
Also, end-to-end-encryption is not related directly to IMAP.
The issue with Hey/Protonmail/Tutanota is that they do not offer IMAP- or other standard-protocol-support - so you can just not log in with Delta Chat (nor with any other e-mail-client) to your Hey/Protonmail/Tutanota account.
If you log in to deltachat say with Posteo, you can write to other Hey/Protonmail/Tutanota accounts - but as the recipient cannot use Delta or other Autocrypt capable clients (Enigmail, K9), things cannot be encrypted. If Hey/Protonmail/Tutanota would support Autocrypt in their webclients, of course, this can change.
Here is a list of tested providers:
https://providers.delta.chat/