/**
 * Fully Disable Gutenberg editor.
 */
add_filter('use_block_editor_for_post_type', '__return_false', 10);
// Don't load Gutenberg-related stylesheets
add_action( 'wp_enqueue_scripts', 'remove_block_css', 100 );
function remove_block_css() {
wp_dequeue_style( 'wp-block-library' ); // WordPress core
wp_dequeue_style( 'wp-block-library-theme' ); // WordPress core
}