Hello John, this is Emily.
Microsoft did indeed end the free upgrade from Windows 7 to 10 back in 2016. However, the upgrade path was never removed until last month. This is why many claimed that they could continue to upgrade without problem. Here is the official public release: https://devicepartner.microsoft.com/en-us/commu...
To clarify, you cannot use the original Windows 7 OEM key to activate Windows 10. If you have claimed the free upgrade in the past from Windows 7 to 10, that process assigned you a digital license instead for Windows 10. This article explains it here: https://support.microsoft.com/en-us/windows/act... And I quote in the screenshot.
So if you did claim the free upgrade before the permanent removal of upgrade path, your Microsoft account at that time is your new license now. So if you are at a step where it asks for product key, select "I don't have any product key", continue your installation. Once installation is done, go ahead and sign into your computer using that same Microsoft account to activate it.
If you have never claimed your free upgrade in the path, then I am afraid you are a bit too late to do so now that Microsoft has permanently removed the upgrade path.