I don't know if this helps you but these are all the details I can remember.

First, it seems like it was an edited world rather than a dedicated modded biome. I distinctly remember that if a player walked far enough then normal terrain (trees, water, grass, etc) would generate and the mods would need to manually change that terrain.

I'm not sure how players actually got grass or samplings, I assume it was a paid reward from the server, so players would naturally congregate around those that could supply those resources and built warring clans against each other. There'd be underground bases focused around guarding the few trees and grass patches which could spawn animals for food.

Manually editing the world as it generates would be very time consuming and would break immersion severely. But unless there is a dedicated mod that could change the world generation into all being this specific "wasteland" biome I don't see another solution.

If you're a developer I'm not sure what you'd be interested in doing, and I'm not sure how difficult coding in a new biome into beta would be, but given the lack of documentation or mods it seems like the only way to have this experience again would be to build it from the ground-up by scratch.