Do a quick Google search and you will come across multiple claims about Headstand (Sirsasana) varying from stimulating the pineal gland and the thyroid gland, to curing bad breath!
It is important to start by saying that good quality research looking at specific yoga poses in very hard to find.
There are only a couple small studies that I have come across that included inversions such as Headstand.
There is evidence that inverting the body causes a transient increase in venous return - the flow of blood back to the heart (Haennel et al. 1988). While there is widespread belief that inverted postures in yoga specifically lower blood pressure by stimulating the baroreceptor reflex (one of the body's homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels), there is very little research on this topic. A few studies several decades ago began to explore this (Razin 1977, Tai and Colaco 1981 and Cole 1989) but it is challenging to find more recent literature that expands upon this topic. So essentially, other than momentarily improving venous return the jury is still out on whether there are specific physiological benefits to be gained from inverting the body.
We then have the practicalities of practising Headstand to contend with. Firstly, while our neck is happy to bear the weight of our head, for most of us our neck has not been prepared to deal very well with much more weight than that. Even with a conscious effort to only bear weight in the arms in Headstand it is almost impossible not to bear a significant amount of weight in the head. For many of us the length of our humerus is much shorter than the height of our head and neck and props are required to supply the extra height. How we transition in and out of the asana is also very important. A study by Hector and Jensen (2015) concluded that entering the asana with straight legs together may reduce the load applied to the head and neck, and the rate of change of that load. Not many of us have the strength and control (yet) to do this.
Should these poses be banned in yoga studios? No, I don’t think so. It can give the false impression that we are all completely fragile. I think it’s a matter of students and teachers getting more educated around these topics. The “FeetUp” chair is a great prop to use for Headstands and a wonderful alternative is to stick to good old Legs-Up-The-Wall (Viparita Karani).
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References:
Cole, R. 1989. “Postural baroreflex stimuli may affect EEG arousal and sleep in humans.” J Appl Physiol 67(6): 2369-2375.
Haennel, R., Teo, K., Snydmiller, G., Quinney, H. and Kappagoda, C. 1988. “Short-term cardiovascular adaptations to vertical head-down suspension.” Arch Phys Med Rehabil 69(5): 352-7.
Hector, R. and Jensen, J. 2015. “Sirsasana (headstand) technique alters head/neck loading: Considerations for safety.” Journal of Bodywork and Movement Therapies 19(3): 434-441.
Razin, A. 1977. “Upside-down position to terminate tachycardia of Wolff-Parkinson-White syndrome.” N Engl J Med 296(26): 1535‐1536.
Tai, Y. and Colaco, C. 1981. “Upside-down position for paroxysmal supraventricular tachycardia.” Lancet2(8258): 1289.