Update: And now it's official: The full Half-Life Collection is, for awhile at least, free to play. "Half-Life: Alyx is coming in March, and we are celebrating early by making all games in the Half-Life Collection FREE to play for Steam users, from now until the day it launches," Valve said in announcement (opens in new tab).
"Half-Life: Alyx is set before the events of Half-Life 2 and the episodes, but the games share characters and story elements. The Half-Life: Alyx team believes that the best way to enjoy the new game is to play through the old ones, especially Half-Life 2 and the episodes, so we want to make that as easy as possible."
The Half-Life Collection includes Half-Life (opens in new tab), Half-Life 2 (opens in new tab), Half-Life 2: Episode One (opens in new tab), and Half-Life 2: Episode Two (opens in new tab), as well as the spin-off games Opposing Force (opens in new tab) and Blue Shift (opens in new tab), and the multiplayer Team Fortress Classic (opens in new tab).
Some Steam users received a notification (opens in new tab) yesterday that advertised a Half-Life: Alyx event before vanishing. The notification claimed that the Half-Life Collection would be free until the launch of the VR spin-off, except there is no such thing as the Half-Life Collection, and right now all the Half-Life games have prices.
While there are no bundles known as the Half-Life Collection, there's The Orange Box and the Half-Life Complete bundles, but neither of them are free. And despite the notification, which links to an error page (opens in new tab), Valve hasn't made any announcements.
It looks like the notification was sent out too early, but that doesn't explain why the bundle has a different name. It could simply be a mistake, so the notification could just be referring to Half-Life Complete. It could also be that Valve's going to release a third bundle, which will eventually contain Half-Life: Alyx.
I've reached out to Valve to find out more about the event, so keep an eye out for updates.