What this would do, exactly, is in the structure element menu, it is an option that could be checked, would be like how village houses used to generate. If a village house tried to spawn on the side of a hill, it would copy the bottom layer all the way down until all blocks in the layer are touching a block that is not air/plants/other non-solid blocks.
And how this would be useful in mods, if someone was to make a structure that is on stilts, like a hut, and the structure spawns on a hill, it won't look bad because the option stretched the stilts down to the ground.
Please add this! :)