https://www.techradar.com/news/windows-10-free-upgrade-is-still-alive-and-kicking-in-2018

Microsoft’s offer to upgrade Windows 7 or Windows 8 to Windows 10 for free for users who need assistive technologies was supposed to run out at the end of 2017, and despite a loophole being open for a while, Microsoft has closed it.

The assistive technologies upgrade was meant for anyone who was going to use Windows 10’s accessibility features, and there was no verification process involved, so anyone was able to upgrade using this method (dubious moral issues aside).