Steam2
Steam2 refers to the Steam previous content distribution system, internally known as Steam2, used until the transition to SteamPipe in 2013.
Game Distributions
Games are stored in the Steam's content servers under several .blob (Metadata) and .dat (Data) files, with the filename typically contains the AppID, version ID, along with random ID at the end. Instead of storing every single gigabytes of different versions of the games which can significantly use more disk space on Steam's servers, Steam2 stores multiple versions of the game differently, with version "0" of an application typically contains the base game files, with anything after that, version "1", "2" and so on, acts like a "patch", which can be few megabytes to over a gigabytes.
Game updates typically requires Valve approval before they can be distributed, this makes sure that developers doesn't push out games that were either non-functional, filled with bugs or being marked as a malware.
When downloading a game, it would be stored on Steam\steamapps\<username>\Game Name folder, as Steam at the time stores game files on separate user account folder (the folder would be named after Steam username), as opposed to multiple Steam users sharing game files under "common" folder. Games that uses GCF (Game Cache Files) such as GoldSrc or Source games would store .gcf files on Steam\steamapps folder. GCF is also the file format used to store GoldSrc and Source engine game (except Left 4 Dead engine branch and later which uses VPK instead and store some of it's game files & binaries directly on the filesystem), engine and binaries files prior to SteamPipe.
Leaks
At some point after Valve has switched from Steam2 to SteamPipe few years ago, multiple leaks of Steam2 content has been occurred, one being Steam2 chunks from 2005-2009 around late 2010s or early 2020s, which contains some versions of GoldSrc & Source games up to 2009.
Later in August 30, 2026, a massive leak occurred, dubbed "Steam2 Teraleak", leading to nearly entire of the Steam2 content server (starting from Steam launch in 2003 up to 2013) being leaked, although most of the content from 2007 and earlier are missing it's actual files. As Steam2 not only contains all versions of a released game builds from both Valve and third-party developers, some developers also stored several in-development/beta versions of a game on Steam. In Valve case, a number of early builds of Left 4 Dead 2, Portal 2, Dota 2, Counter-Strike: Global Offensive, which are stored in several different AppIDs during development, has been leaked.
According to some users on X (Twitter) and other social media, it is presumed that the cause of the leak is likely because of the Valve's Steam2 endpoints which are not only likely left open publicly & unused, but despite the content being encrypted, Valve accidentally may have shipped the keys. The leak may also have been occurred prior to August 30, 2026, spreading though private groups until publicly leaked in August 30, 2026.