Disable Full Site Editing (FSE)

add_action( 'admin_menu', function() {

	remove_submenu_page('themes.php', 'site-editor.php');

});

add_action('admin_bar_menu', function($wp_admin_bar) {

	$wp_admin_bar->remove_node('site-editor');

}, 250);

add_action('admin_init', function() {

	global $pagenow;

	if ('site-editor.php' === $pagenow) {

		wp_safe_redirect(admin_url()); 

		exit;

	}

});