ERBlank: Came across a particular situation with
Leisure Suit Larry: Reloaded: despite missing the Linux build, the game was included into the GOG preservation program. Turns out, the game was de-listed from Steam. While it's regrettable that the Linux version remains unavailable on GOG, I'll remove the entry because technically there's no more a discrepancy between GOG and Steam in that sense.
Curtainferno: Steam keys for it are still available from key shops (both official and grey market). I think it still belongs.
(And even if the Steam version wasn't available anymore, this is still the sort of thing I would want to know about, if I were interested in the game.)
Fair point. I updated the list, but with explicit mention of the possibility to buy the Steam version of the game from other key shops. Because that should be the emphasis of the list, the discrepancy between GOG and other
digital stores. If, for example, non-English localisations of a game aren't available both on GOG and Steam and can only be found in old retail discs, that would no longer be an issue that is unique of GOG, and would be outside the scope of this thread.
VanishedOne: According to IsThereAnyDeal, The Bookwalker: Thief of Tales has an artbook DLC (The Bookwalker: Thief of Tales - Digital Artbook) that's sold only on Steam.
tinyBuild again apparently