Two methods which were detailed back in January 2017 which allowed users to upgrade from Windows 7 or 8.1 to Windows 10 for free still exist now, even though the free upgrade period ended over a year ago. One method involves taking advantage of a program for those with disabilities, while the second approach only needs a valid Windows 7 or 8.1 install and product key.
https://www.notebookcheck.net/Two-methods-of-obtaining-Windows-10-for-free-appear-to-still-be-open.241404.0.html
Honestly, there's nothing surprising that you can install or upgrade with an Windows 10 ISO image and a valid key, even if that key is for 7, 8, 8.1. To allow the upgrades in the first place Microsoft had to authorize all keys for these Windows versions as valid for installing Windows 10, and keeping track of what keys where used or not to upgrade during that period would be a huge waste of resources and wrongly un-authorizing valid keys now will certainly result legal trouble, so they'll never revert it, and every key issued for any version of Windows since 7 will be a valid Windows 10 key, forever. There's no doubt, you can always and forever install the Windows 10 ISO with any previously obtained key for Windows 7, 8, 8.1 and 10, and that's in Microsoft's best interest you do so whenever.
I suspect that Microsoft would also like Windows 10 to have a much larger market share so that people would stop talking about how many systems are still using Windows 7, and so that they would have more monetization options with the store and their various services that are most closely integrated with Windows 10 than Windows 7.