Such an API would help adapting to the internal changes. The fact that GPG Mail no longer works after major updates, is related to the circumstance, that Apple does not provide any APIs for Mail app. This is because for new version of macOS, Apple changes a lot of internals in Mail app. While minor updates are mostly unproblematic, major updates of macOS are likely to break GPG Mail compatibility. For that to work, please ensure automatic update checks are enabled in System Preferences > GPG Suite Preferences > Updates. We do our best to ensure that macOS updates won't disable GPGMail.
