Plugin to make WordPress comments available at <post_url>/comments/.

comments-page.php

<?php
/**
 * Plugin Name:     Comments Page
 * Plugin URI:      https://WPSessions.com
 * Description:     Makes comments for a post accessible at <post_url>/comments/.
 * Author:          Brian Richards
 * Author URI:      https://rzen.net
 * Text Domain:     comments-page
 * Domain Path:     /languages
 * Version:         0.1.0
 */

namespace CommentsPage;

// Original props to https://github.com/stormuk/wp-comments-page for the source material.

/**
 * Register endpoint and flush rewrite during activation.
 */
function activation_hook() {
    comments_endpoint_add_endpoint();
    flush_rewrite_rules();
}
register_activation_hook( __FILE__, __NAMESPACE__ . '\activation_hook' );

/**
 * Flush rewrite rules during deactivation to remove custom endpoint.
 */
function deactivation_hook() {
    flush_rewrite_rules();
}
register_deactivation_hook( __FILE__, __NAMESPACE__ . '\deactivation_hook' );

/**
 * Register the /comments/ endpoint for all post permalinks.
 */
function add_comments_endpoint() {
    add_rewrite_endpoint( 'comments', EP_PERMALINK );
}
add_action( 'init', __NAMESPACE__ . '\add_comments_endpoint' );

/**
 * Make 'comments' an accessible query_var.
 *
 * @param  array $vars Registered query_vars.
 * @return array       Updated query_vars.
 */
function add_comments_query_var( $vars ) {
    $vars[] = 'comments';
    return $vars;
}
add_filter( 'query_vars', __NAMESPACE__ . '\add_comments_query_var' );

/**
 * Change the template used when the permalink includes /comments/.
 *
 * @param string $templates
 * @return string
 */
function register_comments_template( $templates = '' ) {
    global $wp_query;
    
    if ( ! isset( $wp_query->query['comments'] ) ) {
        return $templates;
    }
    
    $templates = locate_template( 'single-comments.php', false );

    if ( empty( $templates ) ) {
        $templates = dirname( __FILE__ ) . '/single-comments.php';
    }
    
    return $templates;
}
add_action( 'single_template', __NAMESPACE__ . '\register_comments_template' );

/**
 * Update individual comment permalink to include endpoint.
 *
 * @param string     $link    The comment permalink with '#comment-$id' appended.
 * @param WP_Comment $comment The current comment object.
 * @param array      $args    An array of arguments to override the defaults.
 * @param int        $cpage   The calculated comment page value.
 */
function get_comment_link( $link, $comment, $args, $cpage ) {
    return get_comments_link( $link, $comment->comment_post_ID ) . '#comment-' . $comment->comment_ID;
}
add_filter( 'get_comment_link', __NAMESPACE__ . '\get_comment_link', 10, 4 );

/**
 * Update comments link to point to comments permalink page.
 *
 * @param string $link    The current comments link.
 * @param int    $post_id The current post ID.
 * @return string         Updated comments permalink.
 */
function get_comments_link( $link, $post_id ) {
    return untrailingslashit( get_permalink( $post_id ) ) . '/comments/';
}
add_filter( 'respond_link', __NAMESPACE__ . '\get_comments_link', 10, 2 );
add_filter( 'get_comments_link', __NAMESPACE__ . '\get_comments_link', 10, 2 );

/**
 * Update the page title when viewing a comment permalink.
 *
 * @param  string $title
 * @return string
 */
function update_page_title( $title ) {
    global $wp_query;

    if ( isset( $wp_query->query['comments'] ) ) {
        $title = sprintf( __( 'Comments for %s' ), $title );
    }

    return $title;
}
add_filter( 'wp_title', __NAMESPACE__ . '\update_page_title' );

single-comments.php

<?php get_header(); ?>

	<div id="primary" class="content-area">
		<main id="main" class="site-main" role="main">

            <h1><?php printf( __( 'Comments for <a href="%1$s">%2$s</a>', 'comments-page' ), get_permalink(), get_the_title() ); ?></h1>

            <?php
                if ( comments_open() || get_comments_number() ) :
                    comments_template();
                endif;
            ?>

		</main><!-- #main -->
	</div><!-- #primary -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>