While Killer Instinct hasn't even been released on Steam, the latest update for the game opens it up to those playing on Valve's platform. The feature might be toggled on and off after it first deploys, which is just the developers tinkering with it in the background to work out the kinks, according to an Xbox blog post. Even if they close off play for Steam users temporarily, they'll keep it on for in-network (i.e. Xbox One and Windows 10) players.
It's a neat move, and perhaps independent PC platforms are the right partners to start knocking down the walls separating siloed systems. If the recent Ark: Survival Evolved kerfuffle is anything to go by, Sony still won't make nice and enable fans to play with other systems.