10.14.5 should be released "real soon now".
A notarized app will work on any OS version that meets the minimum deployment version.
It is safe to assume that a Notarized app will still be notarized in future versions of the OS. However, 10.15 is also due out soon, so there may not ever be a 10.14.6.
For me, notarization takes approximately 2 minutes.
Hence deployment targets can't go earlier than 10.9.
That’s not quite right. I’ve personally built and notarised an app and run it on 10.7.
The 10.9 restriction relates to the SDK. To notarise an app you must build it with the 10.9 or later SDK. However, it’s absolutely supported for folks to set the deployment target version less than the SDK version. In my case, I built my app with the 10.14 SDK and the deployment target to 10.7.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"