https://en.wikipedia.org/wiki/MacOS has a footnote after the build number. That led me to https://developer.apple.com/news/releases/. I guess that doesn’t do semantic markup, and I didn’t look at the html at all, but it looks like it could provide you what you want fairly easily (likely not 100% reliably if automated, but chances are Wikipedia‘s somehow keeps up to date involves humans, too)

Alternatively, buy a Mac, set it to auto-update, have cron or launchd reboot it reboot it twice a day, and read the version info from the CLI after rebooting (https://coderwall.com/p/4yz8dq/determine-os-x-version-from-t...)