Aperture Desk Job, as the spin-off is called, is an interactive tutorial that can be downloaded from 1 March. It introduces owners of the brand new Steam Deck to the various possibilities. The spin-off takes about twenty minutes to complete.
Valve emphasizes that it is absolutely not about Portal 3 goes. When and if that will ever come is not clear at the moment. Aperture Desk Job can be downloaded from this link. Below the trailer.