Previously, only nodes strictly within the map block were being carved out as open space. This meant that multi-node decorations (such as giant mushrooms) would collide with walls and ceilings that would be carved out by adjacent block generation later, leaving them sliced off.
Now Subterrane carves out border regions too. Make note of the is_ground_content function, put all your decoration nodes into this to prevent them from being removed. Can't rely on just is_ground_function node properties for most situations, it's too simplistic.
This major rewrite grew out of a desire to be able to have stalactites placed according to a global noise function, which meant the whole decoration approach had to be changed to allow for data to be efficiently carried over between nodes. In the process I cleaned up a lot of old inefficient code, some of it written back when I was still learning LUA in general.
The legacy.lua file contains the old deprecated code so in theory any mods depending on subterrane should still function as they did before.
