WDDM isn't an API. WDDM doesn't talk to a application to send it off to perform task, or make it really have much functionality at all. It's the minimum needed by the GPU to send framebuffer/drawcalls into, to which DirectX and various other API's can be built on. Think if it as a set of minimum rules laid out by the OS, not something to help interface to a program, but rather more lower level driver side which is total controlled by whomever makes the display driver.
you linked to 384.69 FORCEWARE released on 2017.8.22. Scroll up and see exactly what I said. This driver is hardly from 2011, its from this year. Try reading the change log from the legacy selection and installing it on a modern system. Listen What im saying is thanks to NDIS, WDF which are components in windows, generally you can use OLDER driver versions on newer Operating systems. Lets say you have a older sound card, and the drivers are only made for windows 7 (KMDF 1.11), generally they will work on Windows 10, I've tested Windows 10 on older hardware drivers. They work, generally, sometimes they are buggy and can cause the system to crash because the old driver generally isn't tested/designed around the newer OS.
Linux is just a kernel, as you mention to make a distro or a complete desktop usable friendly system, you need some kind of display manager, window manager, bootloader, complier, package managment system, to which there are ALOT of choices, and to a person who designs a binary, they have to have some dependencies, which is why linux isn't the best at running binarys. All these things are not standard in a linux. Sure most distro's tend to install X, but Linux (as a distro) changes faces all the time. Look at init vs systemd and the debian debate with Debain, and other people trying to switch from X or Mir to Wayland, and Nvidia still not budging much from wanting to only support Xorg and not others.
What I'm saying is windows, hate it or love it, generally is better for compatability because of yes it's wide user base based on marketing and economics, but particular the reason why linux never set off on the desktop for programmers Is because they didn't want to open source their software and with linux it's dependency hell when you shift through what user base has what installed, That's why android took off on smart phones, is google/sun-java/etc provided a set of common tools that evolved over time, but still programmers could make their apk's and binary's to generally have it work on a wide array of devices. They didn't have to focus on anything going on the user-space and mostly targeted java, and now obviously they built a thriving user base out of making popular and keeping with the same packages and user space on different phones.
I'm not saying linux is worse because it has options, im simply saying from a person that just doesn't want to release the source code of said program, linux as in a desktop distro is hard to design for and predict long term. Sure, linux is much more advanced and in some respect better than faster and more robust than windows, but again back to pro's and con's. What one dev see's a pro can be a con to some.
No, and I don't pretend to know. They don't have to do it twice, they just have to check if the version they made on WDDM 2.x works on 1.3, They still made Drivers for Win7 (1.2) so they still check for backports, it's ABSOLUTELY the fault of the PERSON who makes the GPU to discontinue support and SOFTWARE for support, its their decision, Microsoft isn't forcing them to discontinue their support, they ultimately have that decision to make 3 similar but seperate and test them, they don't want to, yet NVIDA is making driver support as you noted for Linux kernels 2.6.9 and Win7-win10.
My point of the original rant was Win7, Win8, and Win10 ARE still supported and used by the bulk of gamers, excluding one just because it has a smaller userbase because you can't test compatibility, fine, you're a company, you have bills to pay. But remember I am a consumer, and I vote with my wallet, and Nvidia is clearly supporting CARDS WAY longer than AMD. I will vote with my wallet next GPU purchase. That was really the point of this, and to give some technical overview
