Quoted from the http://tidbits.com/article/17633: "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." [...]
(Score: 3, Informative) by Rich on Tuesday November 21 2017, @01:25PM
I have a 2009 MBP as "daily driver" which
can't runis excluded from updates to newer than 10.11. Looks like that's a feature now, which can save a lot of hassle :) (though mine, downgraded to 10.9, still keeps nagging me for upgrading iTunes to a crappier version...). Time to re-iterate once more that it continually went downhill past Snow Leopard.