Source

File: wp-includes/class-wp-query.php

public function set_404() {
		$is_feed = $this->is_feed;

		$this->init_query_flags();
		$this->is_404 = true;

		$this->is_feed = $is_feed;

		/**
		 * Fires after a 404 is triggered.
		 *
		 * @since 5.5.0
		 *
		 * @param WP_Query $this The WP_Query instance (passed by reference).
		 */
		do_action_ref_array( 'set_404', array( $this ) );
	}