Is there a way to list all used/unused WP templates?

Here’s a rough function I use to handle this, it should work well for anyone out there wanting to do this quick and easy. Add this this your functions.php file and then visit your site with ?template_report added onto the URL to display a report for every custom theme template.

It’s rough, I’d suggest commenting/uncommenting the add_action call when you want to use it.

/**
 * Theme Template Usage Report
 *
 * Displays a data dump to show you the pages in your WordPress
 * site that are using custom theme templates.
 */
function theme_template_usage_report( $file = false ) {
    if ( ! isset( $_GET['template_report'] ) ) return;

    $templates = wp_get_theme()->get_page_templates();
    $report = array();

    echo '<h1>Page Template Usage Report</h1>';
    echo "<p>This report will show you any pages in your WordPress site that are using one of your theme's custom templates.</p>";

    foreach ( $templates as $file => $name ) {
        $q = new WP_Query( array(
            'post_type' => 'page',
            'posts_per_page' => -1,
            'meta_query' => array( array(
                'key' => '_wp_page_template',
                'value' => $file
            ) )
        ) );

        $page_count = sizeof( $q->posts );

        if ( $page_count > 0 ) {
            echo '<p style="color:green">' . $file . ': <strong>' . sizeof( $q->posts ) . '</strong> pages are using this template:</p>';
            echo "<ul>";
            foreach ( $q->posts as $p ) {
                echo '<li><a href="' . get_permalink( $p, false ) . '">' . $p->post_title . '</a></li>';
            }
            echo "</ul>";
        } else {
            echo '<p style="color:red">' . $file . ': <strong>0</strong> pages are using this template, you should be able to safely delete it from your theme.</p>';
        }

        foreach ( $q->posts as $p ) {
            $report[$file][$p->ID] = $p->post_title;
        }
    }

    exit;
}
add_action( 'wp', 'theme_template_usage_report' );

The output looks like this: