Apple’s most meaningful communication about this inexcusable failure of support were the 404 errors from its download pages.
You’re running High Sierra, but SystHist reckons you installed Mojave, and have recently applied High Sierra security updates. Isn’t it wrong?
When installing the 10.14.3 update, Software Update’s progress bar mislead, and the process became confusing.
Now recognises 10.14.1 update fully, and is notarized for your added security. Recommended for all users.
There’s a very long list of macOS features which are now updated automatically. Advice on how you should configure the Software Update pane and more. Look out for macOS 10.14.1 soon.