What class was created in El Salvador due to industrialization and urban growth?

In general, the class created by these conditions is an urbanized underclass.  This happens as people come to the cities in search of work.  If they cannot find work, or if the work does not pay well enough, they end up poor and, often, living in shanty towns.

El Salvador has not had as much of this as other countries like, for example, Brazil.  El Salvador's industrialization was not as robust as other countries' and so there was less to draw Salvadoreans to the cities.

Even so, urban growth did happen and did create an underclass living in shantytowns.