remove-all-thumbnails.php

<?php

/*
* CAUTION!
* This code will delete all generated thumbnails. Useful if you want to regenerate.
* Don't forget to regenerate thumbnails with wp-cli or any plugin.
*/

add_action( 'init', 'remove_all_thumbnails' );

function remove_all_thumbnails(){
	$attachments = get_posts( array(
		'post_type' => 'attachment',
		'posts_per_page' => -1,
	) );

	foreach ($attachments as $attach){
		$filePath = get_attached_file($attach->ID);
		$ext = pathinfo($filePath, PATHINFO_EXTENSION);
		$dirname = pathinfo($filePath, PATHINFO_DIRNAME);
		$filename = basename($filePath,".".$ext);

        foreach (glob("{$dirname}/{$filename}-*") as $delete_file) {
            unlink($delete_file);
        }
	}
}