I am on 8.1, problem is I tried upgrading to windows 10 a year ago. I was one of the "early adopters". I cant remember properly but there was incompatibility with Visual Studio with Xamarin on. I was reliant on developing an app at that time and could not afford downtime, so I reverted back to 8.1.
8.1 still gets updates until 2023. They only mean 8.0 with those packages. :)
They should add a clearer error message when trying to install an 8.0 package though.
That's news to me. The free upgrade period is over[1].
The only free Windows 10 upgrade is the one for users of assistive technologies[2]. Of course, you can just pretend to be a user of assistive technologies, but I'm uncertain about the legal ramifications of doing so, regardless of whether or not you'll be caught.
Additionally, Windows 10 silently accepts Windows 7 and 8 product keys[3], but the legal situation is equally nebulous in the face of [1].