[WordPress] How to filter out posts that don't have a specific meta key before rendering them to the front-end.

1-exclude-posts-with-meta-data.php

<?php

add_action( 'pre_get_posts', array( $plugin_public, 'acme_exclude_this_post' ) );
/**
 * Excludes all posts from the main feed that have the 'Exclude Post From Index' meta data set.
 *
 * @param  object    $query    The query for retrieving the posts for the homepage.
 * @return object    $query
 */
public function acme_exclude_this_post( $query ) {

	if ( $query->is_main_query() && $query->is_home() ) {

		$query->set(
			'meta_query',
			array(
				array(
					'key'     => 'exclude_post_from_index',
					'compare' => 'NOT EXISTS'

				)
			)
		);

	}

	return $query;

}