It's clutter and we should show some restraint before adding new types.

Enthought's Python Distribution isn't shy about adding tools when they have a demonstrated need.  Likewise, Django adds any reusable tools they need.  Neither have a dict proxy.  I've never run into a use case for it and haven't seen any feature requests for it.   PyPi doesn't have package for it (at least not one with any uptake).  The languages doesn't *need* this, nor do the other implementations which are trying to keep up with CPython.

Guido may let you put in it, but that doesn't mean you should.  Again, I ask you to show some restraint.  The language has already exploded beyond what fits into most developer's heads (even core devs don't even know about many the newer features).