So I got Voidborne, and have upgraded this Habitat to max, even built all eight Habitation Districts on it, yet it only has 4 building slots? Have they been nerfed badly or am I looking at a bug?
I am 99% sure the dev wasn't talking about the voidborne buff specifically there. Yes they can't get the full 12 now, but they should still get the +2 from voidborne, which they currently do not in 3.0.1. Which should be a bug considering this line from the 3.0.0 patch notes.
* * Building Slots are now unlocked through expanding the infrastructure of a colony by constructing City, Hive, or Nexus Districts, by upgrading the capital building of the planet, or through civilian infrastructure technologies. (Note: Habitation Districts on habitats do not. Building slots can be acquired on habitats through the Voidborne Ascension Perk.) The Functional Architecture civic now also grants 1 building slot.
I found that if you click away, then back to the habitat, the additional two buildings are showing. I took 'Functional Architecture for +1 slot, voidborne +2 slots, and researched the next level of capital +1 slot, and Ceramo Metal Infrastructure +1 slot'