I think untouchabilities are social evil because
In the Indian constitution it is written as all the people are treated equally means there is no partiality among the people and every one gains the same right given.
Untouchability are the earlier practices which now we don't follow it now and also considered as social evil now which includes cleansing process like bathing,sprinkle water etc,.
Untouchability in india, is a practice where a particular community considers even touching another community person as polluting one.
It is social evil because by practising untouchability backward classes are deprived of the same rights which the other classes enjoy