It isn't a S/T jutsu, it is just a burst of speed. You also need to gather chakra into your legs to use it so it can't really be spammed (maybe Shisui was able to spam it due to his speed hence the "of the Body flicker" nickname.) People with a lot of chakra can move even faster and at greater distances using it like Naruto and Ay. Most people can still trace a body flicker from normal people.Well it is classified as a S/T ninjutsu but it's not on the level of FTG because of range.