You’re totally right! They should make a 30 GB version that only has Siptah-specific content and nothing more.
That way nobody can build sandstone, stonebrick, reinforced stone, black ice, or any cultural DLC (e.g. Argossean or Khitan) buildings. Nobody can make iron tools or weapons, or steel, or star metal, or anything that you can find in Exiled Lands. Which is fine, because you also can’t farm black ice or star metal, since those nodes won’t have meshes anymore.
Oh, and no Vanir armor or Darfari armor or any of that stuff. Only Siptah-specific armor. The only problem is that you won’t be able to craft it, because the delving bench requires Exiled Land armors to give you the Siptah-specific armor recipes. But I’m sure you’ll find a way around that.
And I’m sure you’ll be fine without horses. Or any mounts. Because those assets are all shared between Siptah and Exiled Lands. Besides, even if they weren’t, you wouldn’t be able to raise horses in a stable, because it’s not a Siptah-specific placeable.
Speaking of placeables, you’ll be fine without an armorer’s bench or a furnace, right? Just the delving bench should be enough. And you don’t need preservation boxes or dryers or compost heaps or campfires or fish traps or any of that boring old stuff, really.
And since you don’t have any crafting stations, you don’t need any thralls, either. Which is just as well, because you won’t have all those NPCs from the Exiled Lands.
The more I ponder your idea, the more I like it. Finally, a truly hardcore survival experience! I can’t wait for Siptah-only edition of Conan Exiles. And to think it’ll free up so much space on the hard drive, too! Why hasn’t anyone thought of this before?!