That scenario sounds absolutely delicious. The logical extreme of self-imposed challenges and combatting overly plentiful resources.
Yeah, the idea is that "This isn't when the oil reserves have run dry. This isn't when a religious event shakes the world. This isn't when capitalism crumbles and Mad Max style anarchy rules. This isn't even when an asteroid wipes out half the life on the planet. This is 1,000,000 years after that when the atmosphere is thin and the sun is burning larger and hotter. It's not the end of the civilized world, it's the end of the entire world."
Most of the creatures would be metal/magical, like golems, which could survive on significantly fewer resources, or hardened like an evolved rhino, or become poisonous/flammable/explosive as a defense mechanism. Most of your armor will be cloth or leather, painstakingly picked out from scraps, but that's alright because only the biggest enemies can afford to use weapons, much less
metal weapons! Eons of atom smashing and poorly done melting of goblinite has reduce the world to useless slag. Morality is out the window, bones are considered edible by most surviving species and blood is a frequent meal idea. If it's usable, you'd better use it, or else someone will kill you and use it, and use you.
In a nutshell, Scarcity to the furthest extreme.
Can you mod in madness?
No.