I don't think it's completely unlikely. As far as I can speculate, what's been going on is that every Sims 3 expansion pack has been released on Steam (albeit, perhaps a little later in some cases). I would suppose Valve has a deal with EA that forces EA to make the The Sims series available through Steam. I say 'force' because I have a feeling that EA doesn't quite like this.
Even though the expansions are available on Steam, they have never been advertised (Store front-page for example). I think that was a move from EA to bolster Origin-based purchases.
Of course, I could be argued that the deal is only valid for The Sims 3, and so The Sims 4 will not be available on Steam. It's possible, but I still think it's a series deal, because The Sims Medieval is also on Steam, and it's not related to The Sims 3. My guess is that The Sims 4 will be available through Steam, but will follow the standard of not being advertised. And, as @DungeonEngineerAlaxtis said, the Deluxe edition might be Origin-only.
UPDATE #2: Eclipse will be available for purchase on 16th September, 2016 on Steam and Google Play. IOS version will ba available about a week later as an update. UPDATE: The game's store page is currently available on Steam, so instead of clicking the Greelight link you can follow this one to access it directly. Is the Mac version of Borderlands going to be available on Steam? Update Cancel. Ad by NetSuite. When will The Sims 4 be available on Steam? Why are there more games available on the PC than on Mac on Steam? Is the original Half life: Source available on mac?