>>>https://support.microsoft.com/en-us/help/13853/windows-lifecycle-fact-sheet
>>
>>Irrelevant
>
>But when does Microsoft stop giving you security patches for it? Isn't that when mainstream support ends? Or am I mistaken about that? Because that would be pretty darn important I would think...

Depends on what you use. About 99% of the patches are for IE, Outlook, Office stuff and such. Since I never used those (last time I installed Word was in 2003, IIRC), I stuck to my practice of running the SP1 of the previous version of Windowses (I went W7 in 2014 :), and accepting no updates at all. Lived without problems until a month ago.