The change was quietly made without any announcement.
Call of Duty: Modern Warfare 2

Those playing Call of Duty: Modern Warfare 2 through Steam may have just discovered they lost the ability to share the game via Steam's Family Sharing feature. According to findings reported on by Rock Paper Shotgun, Activision quietly disabled the use of this feature just yesterday without any sort of warning or announcement.

The change was noted on SteamDB.

In case you are unaware, Steam Family Sharing allows users to share their games with family members. Those playing the shared content can earn their own achievements and have their own game progression saved to the Steam Cloud.

A reason for this feature being disabled for Modern Warfare 2 has not been provided by Activision or developer Infinity Ward. Some assumptions could probably be made as to the reasons why and they would probably be correct. The prevailing theory is that Steam Family Sharing made it very easy for cheaters to just share their account repeatedly without worry of ever being completely locked out of the game.

Right now, Call of Duty: Modern Warfare 2 sits at a "Mixed" rating on Steam with nearly 37,000 user reviews. Most of the negative reviews come from those who have experienced a myriad of issues with the PC launch that include (but are not limited to): Performance woes, bugs, and crashing. People are also very upset about skill-based matchmaking (SBMM), and having to deal with the aim assist when going up against controller users. As far as the bugs are concerned, Infinity Ward has set up a public Trello page that lists the currently known issues for the game and each game mode.