C6 asked me to comment, but I'm afraid I don't have much more to say than the usual. You won't get goodies like AVX-512 and any brand-spanking new instructions unless you're using the latest OS. You might miss out on bleeding edge power management functionality. You'll run into similar issues if you run a dated Linux kernel, if it matters to you, which it likely doesn't. None of this is at all a deal-breaker. It's ammunition for Microsoft's case in trying to get the world to always adopt their latest OS, though hardly a good enough reason in real terms to prevent people from using older OSes outright. That's just something that they want, for all the obvious reasons. Introducing new instructions and security features, though, are admittedly not somethings that can easily be patched into an OS. Not that they can't be, but it's obviously something that MS would rather not spend time and money on, which is understandable.
On the engineering front, MS starts working with Intel and AMD very early in their product development cycles. In some sense, the CPU architecture, the compiler, and the kernel are all developed in conjunction. This also goes for Apple and Linux. The CPU does not need to be taped out to develop the software. You can bet that AMD and Intel have been as clued into Windows 10 development (and the other OSes) as they needed to be for the past few years. You can safely assume that the development cycle for both an OS from Microsoft and a CPU from Intel/AMD to be in the territory of 3-5 years, from starting as a Powerpoint presentation to becoming a retail product.