what is the funtion of the concept and the reality of “the Untouchable” in the God of small things?

I think the concept of the untouchables is simply hate and human slavery veiled though the lens of religion. The untouchable caste was the lowest caste in Hindu religion. This low stature was used by higher castes to take advantage of them and feel superior.

Pappachi would not allow Paravans into the house. Nobody would. They were not allowed to touch anything that Touchables touched. Caste Hindus and Caste Christians. Mammachi told Estha and Rahel that she could remember a time, in her girlhood, when Paravans were expected to crawl backwards with a broom, sweeping away their footprints so that Brahmins or Syrian Christians would not defile themselves by accidentally stepping into a Paravan's footprint. In Mammachi's time, Paravans, like other Untouchables, were not allowed to walk on public roads, not allowed to cover their upper bodies, not allowed to carry umbrellas. They had to put their hands over their mouths when they spoke, to divert their polluted breath away from those whom they addressed. pg 120