Add Custom Post States to WordPress posts and pages

display_post_states.php

<?php
add_filter('display_post_states', 'pc_custom_post_states', 10 , 2);
function pc_custom_post_states($states, $post) {
    if( ('page'==get_post_type($post->ID)) && ('page-templates/custom-page-template.php'==get_page_template_slug($post->ID)) ) {
        $states[] = __('Custom state');
    }
return $states;
}