First, an advertisement to Windows 10 got baked into Windows 8.1, probably via its update service. Sigh.
It appeared as a tray icon.
The resulting window, which doesn't tell me nearly enough, has "Learn more on windows.com" at its top-right.
That leads to
www.microsoft.com/en-us/windows/windows-10-upgrade?ocid=win10_auxapp_LearnMore_win10
.. which does not exist. [ 1 ] what I saw · what I should have seen
Footnotes