Halo Wars, the 2009 real time strategy title released for the Xbox 360, could be getting a port to Steam in the future. According to a screenshot on Twitter, hence the rumor, the game is running on Steam. Obviously, this isn’t much to go off and could be faked but Microsoft has released games on Steam in the past such as the Age of Empires series and Flight Simulator games.
Halo Wars 2 recently released and some versions of the game came with an updated version of Halo Wars. This could be a sign that Halo Wars: Definitive Edition could become a standalone purchase in the future.
Until any official announcement is made, take this information with a grain of salt.