Key points: Missing API functionalities which block the release of GPG Mail as a Mail Extension
Four of the major functionalities missing in the first official release of macOS Sonoma are:
Entire message data is not always passed to the extension making processing the encrypted message impossible
Reliably encrypted drafts
Support for setting the default state of the sign and encrypt button in compose windows which can lead to dangerous side-effects
Sign and encrypt button could go out of sync with internal state, if keyring changes are detected
Probably a good time to reach out to Apple: https://www.apple.com/feedback/mail.html
Honestly I'm not sure if I fully understand their points:
"Entire message data is not always passed…" - so sometimes it works, while sometimes doesn't? At a glance MailKit seems to have the necessary hooks (MEMessageSecurityHandler) though i have not used it myself so can't say if it works as expected or not.
When they say they'll release a version with 14.1 (or 14.0.1), do they have confirmation from Apple that the said blockers will be resolved in that release? If they need more time to implement this it's fair, but the wording of the 'key points' sounds like there are blockers out of their control, then how could they commit a release date without Apple's commitment?