And windows.

And that's just the platforms which technically enforce it. Linux is essentially the only platform which actually supports raw syscalls, in the sense that it's considered a normal system API.

And Windows, in a sense: not that libc is the interface, but that the assembly-level API is not the interface.

And almost every other OS still in use today, what Linux does is reminiscent from OSes like CP/M and MS-DOS.

And seL4.