Microsoft has no plans on releasing Windows 9. They are going from Windows 8 (8.1) to Windows 10.
They decided to skip Windows 9 because a program may wrongly detect Windows 9 as Windows 95, Windows 98 or even Windows ME. According to Reddit user cranbourne, Microsoft decided to forgo Windows 9 on account of legacy code from third-party apps running checks for Windows 9x and the comment reads:
Microsoft dev here, the internal rumours are that early testing revealed just how many third party products that had code of the form
if(version.StartsWith("Windows 9"))
{ /* 95 and 98 */
} else {
and that this was the pragmatic solution to avoid that.
This is almost like Microsoft's version of Y2K.
The other factors that Microsoft skip Windows 9:
Then what? What's the other theories about 9?