Windows 10 is still windows NT. The NT native API is widely used these days. It would be a huge departure for MS to stop supporting it in future versions of windows.
Most parts of the NT API have never been officially documented, officially supported, stable (in the "won't change" meaning), and the tiny parts that have actually been documented come with caveats that they are susceptible to change. Supporting fork through the NT API forever makes no sense if there are no users anymore. They could continue to do it for no specific reason, just because fork is internally needed by WSL for example, and so because it is easy to export the capability through the NT API, but I really don't see why they would necessarily do that.
