Do typical system call interfaces allow reducing the size of a file (without replacing it with a different inode)?

man -s 2 ftruncate says

DESCRIPTION
   The  truncate()  and  ftruncate()  functions cause the regular file
   named by path or referenced by fd to be truncated to a size of precisely
   length bytes.

...

CONFORMING TO
   POSIX.1-2001, POSIX.1-2008, 4.4BSD, SVr4 (these calls first appeared in 4.2BSD).

it goes on to say that if you use ftruncate you must have opened the file for writing, and if you use truncate the file must be writable.

The open(2) system call accepts the O_TRUNC flag that can reduce file size:

O_TRUNC – If the file exists and is a regular file, and the file is successfully opened O_RDWR or O_WRONLY, its length shall be truncated to 0, and the mode and owner shall be unchanged. It shall have no effect on FIFO special files or terminal device files. Its effect on other file types is implementation-defined. The result of using O_TRUNC without either O_RDWR or O_WRONLY is undefined.

It is frequently used when the program aims to overwrite the content of a file entirely. An example is your shell’s file redirection operator as in command > file.