Pretty sure it still works like JAAHAS has posted.
Although, M$ may have changed the EULA or something, I do recall something about it like a year ago limiting the upgrade for "general public". It may make ideed illegal do do it, I dont really know, have to further research to be sure.
I still use it on the computers I build from time to time.

As I said on previous post, Win8.1 is my recomendation, even if most people said is not good, it really is!
Linux Mint (or other distro of choice, I happen to use Mint) is still the way to go as primary OS IMHO, more so if you don´t like shady corporate stuff.

Update: https://answers.microsoft.com/en-us/windows/forum/windows_10-windows_install/how-to-install-and-activate-windows-10-using-your/d713f9f9-e91f-4ffe-a3c0-7ef9639a0559

The article is from 2016 but was last updated 13 june 2019 and the article is still online so it still might be legal.