So, in 2013 Steam fully transitioned to their Steam3 system, in a process known as Steampipe. This changed their delivery method of games, and as such if you've ever used SteamDB, you know these old Steam3 depots can be downloaded.
Well, Valve didn't transition any of their Steam2 content over to this new system, effectively killing them overnight.
If anyone here has steam installs backed up with any games installed, you most certainly have lost files. The most important files would be what is in your steamapps folder, as that holds all the game content itself. (They came in .GCF files, or .NCF files which were just an index and then all the game files themselves were loose). Another important file is the clientregistry.blob file, kept at the root of the install, tracked what the current app record of Steam was the last time you updated the client.
If you can spare it, archiving the entire client would be great too, but that's a second priority to the actual game content itself.
Thanks :)