Adam Engst, TidBITS:

If you’re running macOS 10.12 Sierra or earlier, and do not want to upgrade to 10.13 High Sierra right now, be careful because Apple has started pushing High Sierra to older Macs and making it all too easy to upgrade inadvertently. In short, if you get a macOS notification asking you to install High Sierra, click the Details button to launch the App Store app, and then quit it.

I am not a fan of the TidBITS headline here, but I do get the point. Though the update notification does call out an upgrade to macOS High Sierra, it does look like most other updates. And as many users do when they are confronted with a license agreement or privacy policy, it is very easy to click Install without reading the details.

This is good to know, worth passing along to folks in your community running older Macs.