I have used that to reinstall Windows 10 on my mum's computer once but that already had Windows 10. Really good thing anyone can download that.

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.