Just received this email from Apple today:
We’re following up on our previous email announcing that you’ll soon need to transition to the notarytool command-line utility (included with Xcode 13 or later) in order to continue notarizing your macOS software with Apple. Notarizing software with altool was deprecated in Xcode 13, and the Apple notary service will no longer accept uploads from altool as of November 1, 2023. Existing notarized software will continue to function properly.
For information on notarizing your apps, read [TechNote TN3147: Migrating to the latest notarization tool]https://developer.apple.com/documentation/technotes/tn3147-migrating-to-the-latest-notarization-tool?changes=_3_3.
FYI - “Notarytool” requires Mac OS 13 (Monterey).