There's a new node in town and it's funky: Ray Portal is like the Transparent BSDF node, but it allows you to modify coordinates and directions to achieve... portals! Check out this explanation by Default Cube.
From the Blender Manual:
The Ray Portal BSDF node transports rays that enter to another location in the scene. It can be used to render portals for visual effects, and other production rendering tricks.
It acts much like a Transparent BSDF: render passes are passed through, and it is affected by light path max transparent bounces.
