Is it widespread in law firms?

Very very common atleast in the historical law firms of India.

Yup.

Not common.

quite common especially in chennai

Chennai doesn't have regional lobbies like in Bangalore or Mumbai because there aren't any proper national law firms.

Even the ones who claims have a presence (Khaitan, SAM and JSA) operate by hiring local Tamil lawyers and firms. For eg. Khaitan is a local Litigation office (assentio legal).

It's rare to see non tamilians working in Chennai. Salaries are also low.

Regional lobbies can only exist when there is diversity and multiculturalism.

Quite common. All the Hindi speakers used to hang around together, and there was subtle favoritism that went against people from the non-Hindi states.