Not sure how this link proves the point you're trying to make. I can in fact still download all the games that no longer have a page on the Steam store.
Sorry for the late reply. Dungeon Siege 1 had an expansion pack called Legends of Arranna which was sold as a bundle at first on Steam. Sometimes later LoR was removed from everyone's library leaving only the base game. You can download/pirate LoR and apply a steam fix, but as far as I'm aware, you either have to pay or pirate for LoR. The link is "grey" legally AFAICT (pirated but nigh-abandoned)