As far as I can see, you added just the ntdll function declarations, not the actual IOCP implementation of std.Io. Zig has a policy to not have wrappers in std.os.windows, unless they are used. I don’t know enough about the dev process to tell if a full implementation would be accepted.
Yes, the complete implementation will he a fairly complex project. If you are interested in tackling that, first discussing the details with the core team would make sense.
If you want to just use evented io on windows, not develop the io implementation, check our my “zio” project. I use regular winapi APIs, not ntdll.