Only the first three games had linux builds. Blackwell: Deception ships with agsteam.dll ...
Ah, alright, the stubs are registered for "agsteam" plugin name, so only problem is that function names are different for some reason.
Are you suggesting I should edit AGS so it has some empty function with the name that is missing?
You could try, if you like, as it may be pretty straightforward: find Engine/plugin/global_plugin.cpp and copy a section of code:
if (is_agsteam)
{
ccAddExternalStaticFunction("AGSteam::IsAchievementAchieved^1", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::SetAchievementAchieved^1", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::ResetAchievement^1", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::GetIntStat^1", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::GetFloatStat^1", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::GetAverageRateStat^1", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::SetIntStat^2", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::SetFloatStat^2", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::UpdateAverageRateStat^3", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::ResetStatsAndAchievements^0", Sc_PluginStub_Void);
ccAddExternalStaticFunction("AGSteam::get_Initialized", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::get_CurrentLeaderboardName", Sc_PluginStub_NullStr);
ccAddExternalStaticFunction("AGSteam::RequestLeaderboard^3", Sc_PluginStub_Void);
ccAddExternalStaticFunction("AGSteam::UploadScore^1", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::geti_LeaderboardNames", Sc_PluginStub_NullStr);
ccAddExternalStaticFunction("AGSteam::geti_LeaderboardScores", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::get_LeaderboardCount", Sc_PluginStub_Int0);
ccAddExternalStaticFunction("AGSteam::GetUserName^0", Sc_PluginStub_NullStr);
ccAddExternalStaticFunction("AGSteam::GetCurrentGameLanguage^0", Sc_PluginStub_NullStr);
ccAddExternalStaticFunction("AGSteam::FindLeaderboard^1", Sc_PluginStub_Void);
}
...and replace all instances of "AGSteam::" with "Steam::".
If that works we may add this to main repository too

.
Or later when I have more spare time I could check out the game myself.
I've read elsewhere that it was made with an older version of AGS, but I could not figure out any way to determine which version I would need. It was released on 2011 Oct 12.
When you run the game, engine should print something like "Game data version: NNN" and "Compiled with: AGS X.X.X" to console, which may give at least approximate idea.
But, I dont think knowing this will help to solve this problem, as older engines did not have any Steam stubs at all (they were added much later).
There was also a Steam stub plugin, created by AGSSteam author (
https://github.com/monkey0506/ags2clientstub), but I've got a suspicion that it may have exactly same problem of having newer function names. I dont know if there's an older version of that.