To my knowledge, you were never able to leave dirt behind after smoothing over boulders. It just left behind a rock floor on which nothing could be planted.This is half correct, but you could then build some thing like a wall on that tile and then remove it, which leaves the tile as what ever the base layer is at that place. That's usually what ever soil is around there, but I think maybe on mountain biomes it might leave stone?
To my knowledge, you were never able to leave dirt behind after smoothing over boulders. It just left behind a rock floor on which nothing could be planted.This is half correct, but you could then build some thing like a wall on that tile and then remove it, which leaves the tile as what ever the base layer is at that place. That's usually what ever soil is around there, but I think maybe on mountain biomes it might leave stone?
Given that you're using the LNP you could use DFHack's tiletypes (https://docs.dfhack.org/en/stable/docs/tools/tiletypes.html) to paint soil in that place instead.