All the Ways You Can Still Upgrade to Windows 10 for Free - MaslowCNC/GroundControl GitHub Wiki

At the time of this posting - 12/28/17 this information is still valid

https://www.howtogeek.com/272201/all-the-ways-you-can-still-get-windows-10-for-free/

FYI - I have 2 computers active where I downloaded the Windows 10 ISO and chose to install Windows 10 Pro - they both show a you need to register watermark and continue to operate with no restrictions. Note these are new clean installs, not upgrades.

Proceed with the knowledge this is a gray area.