Wordpress Disable Comments (add to function.php)

function.php

<?php

add_action('admin_init', function () {
    // Redirect any user trying to access comments page
    global $pagenow;
    
    if ($pagenow === 'edit-comments.php') {
        wp_redirect(admin_url());
        exit;
    }

    // Remove comments metabox from dashboard
    remove_meta_box('dashboard_recent_comments', 'dashboard', 'normal');

    // Disable support for comments and trackbacks in post types
    foreach (get_post_types() as $post_type) {
        if (post_type_supports($post_type, 'comments')) {
            remove_post_type_support($post_type, 'comments');
            remove_post_type_support($post_type, 'trackbacks');
        }
    }
});

// Close comments on the front-end
add_filter('comments_open', '__return_false', 20, 2);
add_filter('pings_open', '__return_false', 20, 2);

// Hide existing comments
add_filter('comments_array', '__return_empty_array', 10, 2);

// Remove comments page in menu
add_action('admin_menu', function () {
    remove_menu_page('edit-comments.php');
});

// Remove comments links from admin bar
add_action('init', function () {
    if (is_admin_bar_showing()) {
        remove_action('admin_bar_menu', 'wp_admin_bar_comments_menu', 60);
    }
});

Nice work by this thread, so here's an additional improvement you might like.

I noticed 5 queries on wp_comments were running for each admin page, which can be removed. I put this in the "init" hook where I already have code inside an is_admin() test, but you don't need this test. Also, the filter can't go in the "admin_init" hook as that is too late as the queries will have already run.

add_action('init', function () {
	if (is_admin()) {
		// stop queries to count comments, which are called when the admin menu is displayed
		add_filter('wp_count_comments', function() {
			return (object) [
				'approved'       => 0,
				'moderated'      => 0,
				'spam'           => 0,
				'trash'          => 0,
				'post-trashed'   => 0,
				'total_comments' => 0,
				'all'            => 0,
			];
		});
	}
});

You can disable comment feed with:

add_filter('feed_links_show_comments_feed', '__return_false');

To hide the default first comment from the activity feed

add_filter( 'the_comments', '__return_empty_array' );

also

// Return a comment count of zero to hide existing comment entry link.
function zero_comment_count($count){
	return 0;
}
add_filter('get_comments_number', 'zero_comment_count');

can be shortened to:

add_filter('get_comments_number', '__return_zero');

Thanks folks :)

It's better to use this code taken from plugin Disable Comments (mu plugin version). Most up-to-date code.
https://gist.github.com/doiftrue/ab931c1d866cb113b4ff318a5faeb3b3