Slain: Back from Hell have in-game achievements, but after you unlock them it's impossible see them again, unless you use also GOG Galaxy.

That's weird, can you elaborate? (Puoi spiegarti meglio per favore? Il gioco non ha un elenco di obiettivi da consultare nel gioco?)
Shadwen should have in-game achievements, but I'll wait for a confirmation. No idea about Shadowgrounds.

There is no such thing as "in-game achievements" which are different than Steam and/or Galaxy achievements. What the OP is talking about should be labeled as something else which describes it accurately and avoids muddying the waters with confusion.

Honestly, I don't get it. How would you call something that's labeled as "Achievements" in the game (like in Shadow Warrior or ELEX) while the game doesn't support Galaxy achievements? If you're talking about something that "resembles" what achievements are, like in Indiana Jones, I could agree, because that's "just" a completion rate with challenges that give you various points. But that's how achievements work, usually, and that's why it's on the list.

There is an in-game pop-up when you complete the achievement, but apparently there isn't a list of them in any part of the game. In any case the galaxy achievements are the same.

I see, but if the achievements are not stored locally, I wouldn't call them "in-game" achievements. It's just an in-game notification for Galaxy achievements. It's a weird way to handle this kind of stuff in a game, I should say.

Just a little bump to keep the thread alive. Shadwen still needs confirmation about in-game achievements btw, so if anyone has that game please let us know!

Sure!

DEADBOLT added to the list! Thanks to Alexim

You can add also Tempest to the list!

Sure, will do

Sacred 2 have in-game achievements.

Added!

Into the Breach has ingame achievements; same as FTL, they're used to unlock new mechs and content.