It's free still, it doesn't ask you for a key during the install, it just "works".
In my experience, this only works on computers that have digital product key embedded in the BIOS.
These are generally computers that came with Windows 8 or 8.1 from the OEM.
Also, you can still install 10 with a Windows 7 product key.
Yes, Windows 7 product keys have worked for me too.
I have a laptop where the product key sticker was worn off the bottom, I had to call Microsoft support and ask them to activate it. While that call was a bit time-consuming they did finally provide a secret activation code after asking questions and viewing the computer on a remote control session.