Does Drafts (or will Drafts) support iCloud advanced data protection? On the Apple web site it says…
“Third-party app data stored in iCloud is always encrypted in transit and on server. When you turn on Advanced Data Protection, third-party app data stored in iCloud Backup and CloudKit encrypted fields and assets are end-to-end encrypted.”
Advanced Data Protection in not really an app-level feature. It’s system-level. There’s nothing an app needs to do to support it.
If you enable it, any app’s data stored locally on your device will be encrypted when backed up to iCloud.
Drafts does not support CloudKit encryption, which is something totally separate from Advanced Data Protection, but possibly what you are really asking about. Your Drafts data is synced through CloudKit (if sync is enabled) and CloudKit transfers and stores data in an encrypted state, but Apple does have those encryption keys.
More detailed information is available in Apple’s Platform Security guide.
Isn’t Advanced Data Protection essentially the same thing as CloudKit encryption with the later having keys only stored on the user’s device? And when you say that it’s a system-level feature I understand that, but doesn’t the developer have to mark fields in cloudkit as encrypted?
“Advanced Data Protection also automatically protects CloudKit fields that third-party developers choose to mark as encrypted, and all CloudKit assets.” - source
I think the confusion for me is that some of the terms are ambiguous. Speaking way beyond my depth here… 
ADP secures the content of an App using CloudKit Sync only if the code is updated to mark fields as encrypted.
I am not sure how much work this is but some of the other apps have done this (like Agenda and Note Plan). It would be great to have this done for Drafts as well because using a non end to end (E2E) encrypted app for creating (source) content that will end up in an E2E app (like Day One, Agenda, Note Plan, or Omni Focus, all of which are E2E encrypted) defeats the point of having E2E encryption in the destination.
Completely agree with this. Lack of proper E2EE is really holding me back from using Drafts as I’d like to, and if it can be achieved relatively simply via ADP that sounds like a huge win.