Taking a look at SteamDB, the hullabaloo around Half-Life 2's 20th anniversary has sent fans coming back to the single player FPS in droves: With a 24 hour peak of 52,027 players at the time of writing, that's more people playing Half-Life 2 than at any point since Steam started keeping track of concurrent players (or at least the data became available) in 2008.
Now that is a big asterisk, missing both Half-Life 2's original release in 2005 and the launch of the Orange Box in 2007. All the same, it's still an impressive showing for a 20-year-old singleplayer game.
While pure nostalgia and hype from the release of Valve's 2-hour anniversary documentary are definitely big factors, the main driver is likely Valve's beefy new update to Half-Life 2.
The biggest addition, for my money, is the three and a half hours of new developer commentary in the game. Valve has always been an industry leader with its style of in-game creator commentary tracks accessible at little nodes throughout the levels, and I'm eager to find out what new development tidbits are included in this latest batch.
The update has also notably consolidated Episodes One and Two, as well as the Lost Coast demo level, into the base game's launcher.