After reading the Custom WebSocket Handlers documentation and the Router code, I understood that by using WebSocketsRouter::webSocket() a new websocket endpoint is added to the server, that still keeps the default /app/{appKey} route (compliant with the Pusher protocol).

Then, after setting my custom handler endpoint in routes/web.php:

WebSocketsRouter::webSocket('/test', \App\SimpleWebsocketHandler::class);

That just logs that the connection happened:

class SimpleWebsocketHandler implements MessageComponentInterface
{
    public function onOpen(ConnectionInterface $connection) { Log::info('onOpen'); }
    ...

I can verify that the server has both the /app/{appKey} and /test endpoints.

However, when the Laravel Echo instance in the browser tries to connect using the default route:

GET ws://localhost:6001/app/816622f4d65f3c4e1d2e11011658b785?protocol=7&client=js&version=5.0.3&flash=false

The custom onOpen function is executed instead:

Starting the WebSocket server on port 6001...
New connection opened for app key 816622f4d65f3c4e1d2e11011658b785.
[2020-01-30 00:30:51] local.DEBUG: onOpen  
Connection id  closed.

And since the custom handler doesn't establish the connection and nothing is returned to the browser, the Laravel Echo connection fails with the error:

app.js:1 WebSocket connection to 'wss://localhost/app/816622f4d65f3c4e1d2e11011658b785?protocol=7&client=js&version=5.0.3&flash=false' failed: Error in connection establishment: net::ERR_CONNECTION_REFUSED

So, is my understanding wrong? Is it not possible to use both a custom handler and the default Pusher compliant handler together?

In the issue #21 the example provided is a child of the default WebSocketHandler, but then when calling the parent::onOpen(), the Pusher format will be expected.