Well, December 31st has come and gone, and the free offer is still alive and kicking on Microsoft's website. Microsoft has also updated the language on its website to reflect a new termination date:
If you use assistive technologies, you can upgrade to Windows 10 at no cost as Microsoft continues our efforts to improve the Windows 10 experience for people who use these technologies. Please take advantage of this offer before it expires on January 16, 2018.
At this point, Microsoft's deadline for ending the free upgrades for assistive technology users seems to be a moving target. First it was December 31st, 2017; now we're staring down the barrel of a January 16th deadline. However, we wouldn't put it past Microsoft to then extend the offer to March 31st and then push it further and further out.
If you still want to take advantage of free upgrade offer -- and you have a compatible Windows 7 or Windows 8.x system -- click the "Upgrade Now" button on Microsoft's website. Next, a file will download that will allow you to proceed with the installation of Windows 10.
Microsoft announced in November that Windows 10 is currently installed on over 600 million devices around the world. That tally not only includes PCs, but Windows 10 Mobile smartphones and Xbox One consoles as well.