Officially, Microsoft's free upgrade offer ended in 2016, but no one at Microsoft had bothered to reset the activation servers.
So the 25-character product keys from Windows 7 and Windows 8.1 were still valid to activate a new installation of Windows 10.
Microsoft's announcement appeared on 20/09/2023 under this link
"translation below"
"Microsoft's free upgrade offer for Windows 10/11 ended on July 29, 2016. The installation path for obtaining the free Windows 7/8 upgrade is now also discontinued. Upgrades to Windows 11 from Windows 10 are still free."
In short, you can no longer activate Windows 10 and 11 with Windows 7 and 8 keys.
At a time when the company is talking about Windows 12, Microsoft has decided to put a definitive end to this possibility.
However, we don't know exactly how many people will be affected by this change of policy, nor when it will be effective for all PCs, a few days or weeks, with some Internet users mentioning that the transfer was still possible to this day.
Let's not forget that Windows 10 has a 10-year support lifecycle, and that it is scheduled to end on October 14, 2025. And what will happen on that date? Probably nothing.
Computers running Windows 10 will continue to function as they always have, but will no longer receive security updates. Any security flaws discovered after this date will not be corrected, making these computers increasingly vulnerable to attacks...which will certainly be countered by the anti-virus and anti-malware products of the software publishers.