They just released the last expansion about six months ago. But it's not available on Steam. As per the old games, Battle.net is your home for Blizzard games, including digital purchases if that's what you're after.Starcraft II is still out there?
I don't have a Steam account yet (most of my gaming is on the PS4) but if Starcraft is there, I need to go sign up.
Though in my case, I bought Starcraft II on sale from Best Buy for cheap, and it then links to your Battle.net account when you sign in so you can download it on other machines later.