As Katherine says, there are no plans to phase out the Windows version - far from it. Tiho and Lee are working on it constantly, and we are committed to Windows as a platform.
That the Windows version remains behind the Mac version in terms of features is a matter of simple maths, unfortunately. I’m the creator of Scrivener, and I created it on a Mac - and I started coding it back in 2005, eleven years ago. Lee came on board to code the Windows version in 2008, with coding really starting in earnest in 2009, releasing in 2011. So the Mac version had a good three or four year head start. Moreover, Scrivener is not just my job, but my passion (or obsession, I suppose) - I regularly work fourteen-hour days coding it. Lee has had help over the years, these days from Tiho, an incredibly talented programmer, but most of his help has not been full-time, and even if it were, I tend to work two full-time jobs’ worth in one day.
And you have to remember that we are a small company and so don’t have the resources to pay lots of programmers. So all the time Lee was working on the Windows version, I was forging ahead with the Mac version, adding more features, refining, creating a moving target for him, making it very difficult for even two programmers to catch up.
Not only that, but Windows is far more difficult to program for. On the Mac, Apple provides a very solid text system, and the basics of very good import and export. Lee and Tiho had to write a hell of a lot of code that Apple had provided me for free. Even basic stuff like viewing a PDF file involves a lot more work on Windows. This is probably why the independent Mac software scene is thriving - Apple makes it much easier to code for their platform. With Microsoft, it’s even difficult choosing what tools to use, because they tend to change every couple of years.
Lee is very passionate about Scrivener, though, and you have to bear in mind that Scrivener 1.x for Windows has had many major features added for free that Mac users had to pay for in a 2.x update. The Windows version is still missing some 2.x features, but many 2.x features were added along the way without us asking any update price (the next free update brings external folder sync, another 2.x feature). And the Windows version is still cheaper, of course.
Right now, we’re working on major updates for both the Mac and Windows versions. And the plan is still for feature-parity, although the next major Windows version will come some months after the next major Mac version. There’s no avoiding this given that features get coded and tested in the Mac version first, and that the Windows version still has extra catching up to do. We will be very clear in advance about the difference in timings to do our best to mitigate disappointment. The main thing to know is that Scriv for Window is in very active development, and right now, internally, it has all the major features of not only the current Mac version but of the next major Mac version - it’s just that we have a lot more legacy stuff and bugs left to fix and clean up in the Windows version so it will take longer to get there.
So please rest assured that we are very committed to the Windows version. Our Windows team has not been resting on its laurels - far from it. I do understand that Windows users can feel a little neglected when they see that they still don’t have all the features of the Mac version, but we are doing our absolute best as a tiny company. And I do think that Lee has done an amazing job in turning my dream writing app that I created on a Mac into a Windows app - if it wasn’t for Lee, Scrivener for Windows wouldn’t exist at all. There is a lot of cool stuff coming down the line, I promise, but I think that even as it stands it is still one of the best writing apps on Windows. But then, I’m biased. 
The short answer to your question is that the Windows version will almost certainly always lag behind the Mac version a little, simply because I design the app and code the Mac version. But we are working on reducing this lag as much as possible.
All the best,
Keith