Given Valve's recent e-sports focus, this is about as close to closure as we're going to get.

Half-Life 3, gaming's greatest unicorn, may finally be with us—albeit in text form. Marc Laidlaw, the now retired lead writer of the Half-Life series, has published what appears to be a summary of the plot of Half-Life 3, or Half-Life 2: Episode 3, entitled "Epistle 3."

Epistle 3 takes the form of a fictional letter directed to "Dearest Playa" from a character named "Gertie Fremont." It features a ship "Hyperborea," and alien bad guys called the "Disparate," which—when swapped out for original Half-Life names and places—makes the letter as close to a resolution to the series as players are likely to get.

An edited version of the synopsis on Pastebin—which is currently the only place to read it given the unwashed massed of the Internet have taken down Laidlaw's blog—has already replaced the names places and it's ready for reading. The beginning of the synopsis reads:
I hope this letter finds you well. I can hear your complaint already, “Gordon Freeman, we have not heard from you in ages!” Well, if you care to hear excuses, I have plenty, the greatest of them being I’ve been in other dimensions and whatnot, unable to reach you by the usual means. This was the case until eighteen months ago, when I experienced a critical change in my circumstances, and was redeposited on these shores.
In the time since, I have been able to think occasionally about how best to describe the intervening years, my years of silence. I do first apologize for the wait, and that done, hasten to finally explain (albeit briefly, quickly, and in very little detail) events following those described in my previous game (referred to herewith as Episode 2).
Whether Epistle 3 is a final version of the plot or simply Laidlaw's idea of what it might have read like, we'll likely never know. The writer did, however, say in a tweet: "My website's down for now. I guess fanfic is popular, even a genderswapped snapshot of a dream I had many years ago."
With the likes of Duke Nukem: Forever having been released, and Beyond Good and Evil 2 on the way, Half-Life 3 is now the most long-standing vaporware product in games. While Valve CEO Gabe Newell has mentioned the game on and off over the last decade, Valve has forgone any mention of the game in recent years in favour of bolstering its efforts in e-sports games like DoTA 2 and Counter-Strike: Global Offensive, as well as virtual reality where three "full games" are promised. Valve's most recently announced game is Artifact, a DoTA 2 card game.

Whether or not that means the world will never see Half-Life 3 remains to be seen. But Valve's push into e-sports and continued work on the Steam platform over the past few years, while undoubtedly a money-spinner, hasn't left much room for the kind of story-driven experiences the company was once famed for. Valve's last, the sublime Portal 2, was released in 2011.

Laidlaw's Epistle 3 is the first bit of closure for the much-loved series. It's up to Valve to provide the rest—and given it was Half-Life fans that helped turn Valve into the PC gaming powerhouse that it is today, it's about time the company came clean.

Link to Pastebin: https://pastebin.com/q9DMFa7c