The question of whether deadlifts should be done on leg day or not is one that basically comes down to what you’re trying to accomplish. Powerlifters tend to place deadlifts on their own day because of the stress they place on the body. Other lifters don’t mind performing the deadlift on leg day with squats, leg press, etc. In fact, there are even some powerlifters who perform the deadlift on squat day so they can know what it will feel like to perform in a meet. 

It all comes down to the goals of your training session. If your goal is to perform compound movements and touch the hamstrings, quads, glutes, and back at the same time. 

If your goal is to achieve maximal performance in the deadlift then it makes the most sense to perform the deadlift on it’s own day. This is a part of the reason why it makes sense for some lifters to choose training splits that allow them to focus on a single movement (and accessories) on a day.