With this simple plugin you can assign custom pages (with custom content etc.) as error pages. This works for 404 (page not found) and 403 (forbidden/permission denied).

Create custom 401 and 403 error pages with any WordPress theme without writing a single line of code, set it up and forget it.

Instead of serving a 404 Not Found error with the 404.php template, send a 403 Forbidden error and set $wp_query->is_403 and load 403.php if it exists.

Site protection plugin for dev instances to avoid indexing by search engines. Doesn't allow access to the site without being logged-in.