Why is it not necessary to indicate ByVal/ByRef anymore?

It seems that this post covers your question:

http://msmvps.com/blogs/carlosq/archive/2011/03/15/vs-2010-sp1-changing-quot-byval-quot-vb-net-code-editor-experience.aspx

So no, there is no way to get the old behaviour. From now on ByVal is the default (what it was before) and it won't get added automatically to the method parameters.

In my opinion this is a good decision since it's making VB.NET a bit more consistent with C# and avoids unnecessary "noises"(it's already verbose enough).

Old behaviour:

Private Sub test(ByVal test As String)
End Sub

New behaviour

Private Sub test(test As String)
End Sub

Tim covered what you asked directly, but something else to keep in mind is that any reference type variable, like a user defined class even if passed by value will allow you to make changes to that instances properties etc that stay. It won't however allow you to change the entire object. Which may be why it seemed to you to be defaulting to by reference

Public Sub (Something As WhateverClass) 
  Something = New WhateverClass 'will result in no changes when outside this method

  Something.Property1 = "Test"  'will result in an updated property when outside this method
End Sub

From MSDN:

The value of a reference type is a pointer to the data elsewhere in memory. This means that when you pass a reference type by value, the procedure code has a pointer to the underlying element's data, even though it cannot access the underlying element itself. For example, if the element is an array variable, the procedure code does not have access to the variable itself, but it can access the array members.