The patch addressed several persistent issues reported by the community:

If you browse Steam forums or Reddit (r/MW3), you will see three numbers thrown around constantly: 13874, 13680, and 14382. Here is the quick cheat sheet:

Why does this matter for your search? If you downloaded MW3 from Steam today and tried to join a "Plutonium" server, you would get a fatal error: "Version mismatch. Expected 1.4.368, got 1.4.382." You need the 14382 patch to play vanilla Steam multiplayer (if you can find a lobby), but you need to downgrade to play on the healthy, populated community servers.

Old forum guides will tell you to use TeknoMW3. Do not do this. TeknoMW3 was designed for 1.4.382 to enable LAN emulation, but it is dead, insecure, and riddled with exploits. Stick to Plutonium.