What should I do about an exposed wire for landscape lighting?
If it's no longer in use, you really should remove it, or at least disconnect it from the "input" (house electrical) side. Without a fixture present, some person (perhaps even you, if you forget about it) might dig in the yard with no thought of a (sounds like poorly buried if it's exposed, thus probably unmarked as well) wire, and cut into it with a shovel. In which case, it's far better if it's not energized.