Bit of a loaded question really, people mean so many different things when they say Yeast Nutrient.
DAP (Di-Ammonium Phosphate) is sold as yeast nutrient and it is a great source of nitrogen and phosphorus yeast needs both of these. But thats like thinking plants can live on super phosphate and nothing else.
Killed or autolysed yeast will also provide a bunch of things yeast need; bit like the idea that the perfect food for people is pulverised people (odes of Soilent Green) but its hardly a balanced diet.
You can buy Yeast Food there are lots of different versions on the market they are mostly made up of Autolysed Yeast and DAP, but also contain Zinc and a bunch of other Vitamins and trace elements that yeast need. More like giving your yeast Berocca - theyre designed to give the yeast everything it needs cost about 10 cents per brew.
Well made wort should give your yeast everything it requires, but if I ate a balanced diet I wouldnt take a multi pill every morning.
MHB