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