Is the MW2 steam server still online?

Solution 1:

There is some bug with MW2 so the amount of players in game always shows up as 0/0.

If you try to enter a game you should still get in. There is no way of knowing if there is actually anyone in a certain game type so I'd recommend Free-for-all or Team Deathmatch to ensure you get into a game.

Solution 2:

No, Modern Warfare 2 is still a supported multiplayer game. I'm guessing you had some type of network issue or encountered some sort of bug.

Modern Warfare 2 is still among the top 10 multiplayer PC games on Steam at the moment, going by the Steam Game Stats - today its peak players was double that of Black Ops, but nowhere near the amount of players of MW3.

A recent Major Nelson blog post shows that Modern Warfare 2 is still among the most played multiplayer games on Xbox Live as well. (Also note that CoD4 and WaW are still supported games...)

There was some talk of shutting down the Black Ops servers on PSN a while back, which I believe was a reaction to the rampant cheating/griefing that occurred after the Playstation 3 firmware was hacked. However, I do not believe a shutdown actually occurred.