Question

or, why can’t we simply declare pause as:

#include <unistd.h>
void pause(void);

since the return value of pause is meaningless.

Answer 1

From man page of pause

RETURN VALUE

   pause()  only  returns when a signal was caught and the signal-catching
   function returned.  In this case pause() returns -1, and errno  is  set
   to EINTR.

Every syscall which is interrupted is getting EINTR (or ERESTART in peculiar cases). I see no reason to make pause an exception!

Of course you are free to write (void) pause (); in your code to ignore the result.