MacOS Notarization
Notes on MacOS notarization can be found here.
Per the current Electron docs, DMG signing is disabled by default as it will "lead to unwanted errors in combination with notarization requirements." Only the app bundle is zipped and submitted to Apple for notarization.
Extracting Published ZIP Archive in MacOS 10.15 (Catalina)
There is a known bug in which extracting notarized contents from a zip archive via double-click will lead to an invalid .app bundle that cannot be opened in macOS 10.15. Instead, the bundled app should be extracted via ditto:
ditto -x -k <zip archive> <destination folder>
Verification
Notarization status of an application bundle can be verified via the codesign, stapler and spctl utilities:
codesign --test-requirement="=notarized" --verify --verbose WordPress.com.app
xrun stapler validate WordPress.com.app
spctl -a -v WordPress.com.app