clean_post_content.php

<?php
// Warning: this will overwrite every post in your database
// BACKUP FIRST!

clean_post_content();

function clean_post_content() {
    $posts = get_posts(array(
        'post_type' => array('post'), // or page, or cpt
        'post_status' => 'publish', // or any, draft, etc
        'numberposts' => -1, // all posts or number of posts to process
    ));
 
    foreach ($posts as $p) {
        $p->post_content = wp_filter_kses($p->post_content);
        wp_update_post($p);
        echo 'Cleaned post # '.$p->ID.'<br>';
    }
 
    die();
}

Add this script into a page template, apply that template to a new page, save, visit that page, and it will scan through all your posts and apply the format eraser (that eraser button in the editor) to all your posts.