So what IS the state of the art that's also FOSS, perhaps even as-a-library, and non-gen-AI (I'm thinking interactive/game-dev/realtime use-cases)?
Not open source, but Gaia is widely used in games and movies. Can do some cool things like simulated vegetation growth/distribution.
You are probably thinking of gaea - not gaia. Gaia is also a world generator but not as widely recognised as gaea (and gaia is only implemented as a unity plugin IIRC).
Behind several versions of No Man's Sky, let alone whatever madness the LLM people have been cooking.
NMS isn't open source nor for the same purpose
You have to admit the sea water was way better than any video game today. The rest though, yeah.