Drop this file into `wp-content/mu-plugins/` and it will disable login and authentications. Why would you want this? Maybe you need to make an editorial freeze before migrating a site from one host to another, and don't want to risk admins writing content that would get lost in the migration.

wp-freeze.php

<?php

// Usage: Drop this file into `wp-content/mu-plugins/` and it will disable logins and authentication.

namespace WPFreeze;

// In addition, the salts should be temporarily rotated in wp-config.php to invalidate existing sessions.

define( 'WP_FREEZE_MESSAGE', 'This site is currently frozen for maintenance. Please check back later.' );

/**
 * Prevent logins.
 */
add_filter( 'wp_authenticate_user', function() {
    return new WP_Error( 'authentication_failed', WP_FREEZE_MESSAGE );
} );

/**
 * Display freeze message on login page.
 */
add_filter( 'login_message', function( $message ) {
     return '<p class="message">' . WP_FREEZE_MESSAGE . '</p>' . $message;
} );

/**
 * Hide login form.
 */
add_action( 'login_enqueue_scripts', function() {
    wp_add_inline_style(
        'login',
        ' body.login #loginform,
          body.login #nav,
          body.login .language-switcher {
             display:none;
          }'
    );
} );