Should deadlift be done on back or leg day? Is it push or pull? To be completely honest, it can be either. It depends entirely on what your goals are in that particular period. If your focus is on developing a stronger deadlift (and developing stronger back muscles then) you’ll want to use it in pull day. Not to mention that certain phases of the deadlift place more emphasis on the hamstrings (which have been said to act like synergists) while other parts place emphasis on the back muscles. 

It completely depends on what you want out of the movement in any particular instance.