I know HN will see this as Apple further locking down the platform, but it's also plausible that they just want to reduce the number of ways the OS can be installed and upgraded.

Software updates delivered through the automatic updater are listed in an XML catalog file that anyone can parse to get the URL where the actual update package is hosted. The packages usually use the platform standard .pkg installer format.

(Note that the update check does not send information about your system to Apple.)

Apple still provides the `softwareupdate` command line tool to list, download, and apply updates and even whole OS installers of the last 4 years of releases. The OS installers contain tools that can produce bootable install media if you want to archive them.

Apple has also provided tools to mirror software updates on your own infrastructure---see the "Content Caching" option in the Sharing prefpane, for example. Not sure what the state of this is today.

I think the linked article makes good arguments for why it may be premature to discontinue the standalone updates. And I am critical of how difficult it is to obtain OS installers for older hardware and VMs. But it's not the end of macOS as a developer platform.