UWP, slow and steady, will eventually get there.

ChromeOS, the OS APIs are the web platform.

iOS, almost everything that matters is done via Cocoa

Android, Java reigns. There is very little UNIX exposed to userspace, less so since Google started clamping down NDK since version 7.

Not really, but POSIX itself is almost 30 years old (and primarily standardized interfaces that were considerably older), and a lot of interesting stuff happened ca. 1995-2005. See e.g. BeOS or Inferno.