Please, switch your listener to:

  $(document).on('hide.bs.toast',function() {
    alert('ddd');
  });

It will work.

I attach the complete code. According to your API documentation of the TOAST object the event should be captured on "hide.mdb.toast"

ananan

<link href="https://fonts.googleapis.com/css?family=Roboto:300,400,500,700&display=swap" rel="stylesheet"/>
<link href="jquery/MDB8/css/mdb.min.css" rel="stylesheet"/>
<script  src="jquery/jquery-1.11.0.min.js"> </script>

</head>

    <div class="toast-body">Hello, world! This is a toast message.</div>
</div>
<button id="chiudi" class="btn btn-primary">CHIUDI</button>
<button id="apri" class="btn btn-danger">APRI</button>

    <script>

        var tst =null;

        $(document).ready(function(){
            var eltoast = document.getElementById('tst');

            tst = mdb.Toast.getOrCreateInstance(eltoast);
            console.log(tst);




            $('#chiudi').click(function(){
                tst.hide();
            });

            $('#apri').click(function(){
                tst.show();
            });

            $(tst).on('hide.mdb.toast',function() {
                alert('ddd');
            });

        });

    </script>
    <script type="text/javascript"  src="jquery/MDB8/js/mdb.umd.min.js"> </script>

</body>

Please craete a snippet that demonstrates the issue. A HTML part is needed aside of JS. And describe what actions you expect and what you get instead.