Krusty created this very interesting thread: https://community.norton.com/en/forums/optional-update-kb3035583 after which I removed KB3035583, the source of the irritating pop-ups "Get your free upgrade to Windows 10 now!"
I later discovered a new term - Nagware - and created a thread here.
Now there's a development. I was advised to install Soluto - it monitors your start-up times. I'll remove it when I've a moment as I'm not interested in shaving 3.7 seconds odd my power-up time. They've just sent me an email headed "Your Windows is not up-to-date" which says
I'm beginning to believe those who suggested that this KB would be used more widely and more often in the future by Microsoft to push their products on us.