Hi, Og,
"During the launch event (video embedded above) Myerson gives us a few clues. Starting at around the 2:10 mark, he said the following: “We know, based on the product that’s coming, and just how different our approach will be overall, it wouldn’t
be right to call it Windows 9.” He then talks about how Windows One would make sense with Xbox One, OneDrive, and OneNote, “but unfortunately Windows 1 has been done by the giants that came before us.” And so it seems the only other viable option was Windows"
https://www.extremetech.com/computing/191279-why-is-it-called-windows-10-not-windows-9
It was also reported through channels - "Belfiore (who is Corporate VP of Operating Systems at Microsoft) was snapped wearing a special T-shirt at a Build conference. There were four sentences in binary in each quadrant
of the Windows logo on the shirt, one of which ‘explained’ the fate of Windows 9."
It read: “Windows 10, because 7 8 9.”
https://www.itproportal.com/2015/05/07/microsoft-reveals-the-reason-why-there-was-no-windows-9/
Makes one think, doesn't it?