If you leave a fixture in the wilderness for too long, it decomposes. Fixtures can be maintained by repairing them, which anyone can do if they have the right crafting skills.
Stone objects degrade very slowly. They can last for IG years.
Wooden objects degrade at a more moderate rate.
Cushions and couches will only last a few weeks.
Alternatively, the team might be selective about what decays fast and what doesn't. If it's an item that makes more sense only as indoor furniture and doesn't seem appropriate elsewhere, it eventually decays.
This applies anywhere outside of a settlement or quarter. Not just in outdoor areas, but in caves and dungeons too.