Before you proceed, please note that the code below will remove featured images from all posts on your WordPress site by simply just pasting. Also note that this code will not delete any of your uploaded images, they will still be available in Media Library and you can reuse them anytime.
All you need to do is copy and paste this code in your theme’s functions.php file.
global $wpdb; $wpdb->query( " DELETE FROM $wpdb->postmeta WHERE meta_key = '_thumbnail_id' " );
As soon as you save your functions.php file this script will run a database query and remove featured images from all posts.
Important: Please remove this code immediately after saving your functions.php file. You will not be able to set featured images in WordPress as this code will keep removing featured images from posts.