add_action( 'publish_post', function ( $ID ) {
    $post          = get_post( $ID );
    $plain_content = strip_tags( $post->post_content );
    $new_post      = array(
        'ID'           => $ID,
        'post_content' => $plain_content,
    );

    wp_update_post( $new_post );
} );