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It's revelant on servers, workstations, embedded systems, and supercomputers.

So, basically everywhere.

The thing is: you're unlikely to be programming directly on top of the POSIX API unless you're doing something comparatively low level to the average programming that is done in industry today.

Embedded is an area where applications are often written right on top of libc and POSIX interfaces. Even many RTOS which are not fully POSIX compliant borrow a lot of ideas from UNIX.