The breast tax is not a true story. Its complete communist propaganda peddle continuously to circulate the often repeated oppression narrative.
Women in Kerala seldom wore upper garments till 20th century. Even in early 1900s it was common practice. There are enough accounts & depictions of foreign travellers where even the Queen is depicted as such
A 17th-Century Dutch visitor William Van Nieuhoff writes about the attire of Ashwathi Thirunal Umayamma Rani, then queen of Travancore, in the following manner:
"... I was introduced into her majesty's presence. She had a guard of above 700 Nair soldiers about her, all clad after the Malabar fashion; the Queen's attire being no more than a piece of callicoe wrapt around her middle, the upper part of her body appearing for the most part naked, with a piece of callicoe hanging carelessly round her shoulders."
He drew a sketch of the queen and her attendants in his work (Voyages and travels to the East Indies; 1653-1670), where it is quite clear that the queen and her attendants wore little to no cloth to cover their bosoms.
If 17th Century seems a bit dated for our debate, let's look at a few evidences from 19th and early 20th centuries, the very era our fable is supposed to have taken place. The story implies that nakedness was a humiliation imposed by the upper caste on the lower caste women, with the intention of depriving them of modesty and the luxury of wearing a second piece of cloth.
This accusation hardly holds water when you realise that women of Nambuthiri families and affluent Nair families themselves saw no need for a breast-covering garment, either as a sign of luxury or 'modesty'.
As to the specific story of Nangeli and her self-mutilation as protest, there are no historical records of any authenticity of such an event taking place.
A careful look at the references and sources of the Wikipedia article on Nangeli reveal that all of them are from the last decade and mostly from the last couple of years. None of them cites any historical records on Nangeli.
Documentary & film makers got a good opportunity for propaganda and use the subject for showing breasts.
مزید دکھائیں 
