Kernel syscalls themselves are entirely unstable. Not even just the numbers but the calling convention and everything else. Microsoft reserves the right to add, remove or modify anything in or exposed by the NT kernel. Of course they won't do so frivolously but syscall numbers are especially prone to change with a new build of the kernel.
So as with MacOS, BSD, etc the stable interface is provided by userspace libraries.
C headers with the prototypes are available from the Windows Driver Development Kit. It's not something you want to use though, the average kernel function takes 10 arguments to support both sync and async IO.
https://docs.microsoft.com/en-us/windows/win32/api/winternl/...
That remains to be seen in ~10 years at least :-P
