Linux kernel's ABI/API surface is completely byte-based and is tiny compared to Win32 API. The stable ABI of Windows is strictly in the user space and it covers the entire useful operating system. Don't forget that glibc isn't that ABI stable nor anything that goes all the way up to systemd/X11/gettext/Wayland/cairo/GTK/Qt/glade/Pipewire/xdg. Nothing equivalent in Linux environment is stable, especially compared against Win32 system libraries.

You encounter encoding requirements not in kernel system calls but in more user-facing sides of the OS. So your comparison should include all the userspace components of a Linux system (e.g. gettext), if you're considering the places where you encounter actual Unicode string-based operations where UTF-16 comes into play in Windows.

ALL OF THE equivalent Windows libraries (user32.dll, kernel32.dll, ws2_32.dll, shellex.dll ...) to the Linux ones I counted above and more are ABI and API stable.