How to change 404 Not Found response to 410 Gone?

Question

Hi everyone,

I'm trying to change the default 404 Not Found response in Question2Answer to 410 Gone, for some permanently removed pages.

I modified this core file:
qa-include/pages/qa-page-not-found.php

I replaced this line:
header('HTTP/1.0 404 Not Found');

with:
header($_SERVER['SERVER_PROTOCOL'] . ' 410 Gone');

However, when I test the page in the browser, I get a 200 OK response instead 410 Gone.

How can I properly make Q2A respond with a 410 Gone status code instead 404 Not Found?