koel/app/Http/Controllers/Controller.php

<?php

namespace App\Http\Controllers;

use Illuminate\Foundation\Auth\Access\AuthorizesRequests;
use Illuminate\Foundation\Bus\DispatchesJobs;
use Illuminate\Foundation\Validation\ValidatesRequests;
use Illuminate\Http\Response;
use Illuminate\Routing\Controller as BaseController;

abstract class Controller extends BaseController
{
    use AuthorizesRequests;
    use DispatchesJobs;
    use ValidatesRequests;

    protected static function disableInDemo(int $code = Response::HTTP_FORBIDDEN): void
    {
        abort_if(config('koel.misc.demo'), $code);
    }
}