Call them Sudra, Chandala, Nisada, Antya, Bahya, Antyaynis, Harijan, Dalit or down-trodden. They have many classications or nominations as possible.
The Dalit Buddhist Movement has the support of Lanka monks, if I'm not wrong, Bhante Gunaratana did some missionary work with them.
They have 22 vows:
1.I shall have no faith in Brahma, Vishnu and Maheshwara nor shall I worship them.
2.I shall have no faith in Rama and Krishna who are believed to be incarnation of God nor shall I worship them.
3.I shall have no faith in Gauri, Ganapati and other gods and goddesses of Hindus nor shall I worship them.
4.I do not believe in the incarnation of God.
5.I do not and shall not believe that Lord Buddha was the incarnation of Vishnu. I believe this to be sheer madness and false propaganda.
6.I shall not perform Shraddha nor shall I give pind-dan.
7.I shall not act in a manner violating the principles and teachings of the Buddha.
8.I shall not allow any ceremonies to be performed by Brahmins.
9.I shall believe in the equality of man.
10.I shall endeavor to establish equality.
11.I shall follow the noble eightfold path of the Buddha.
12.I shall follow the ten paramitas prescribed by the Buddha.
13.I shall have compassion and loving kindness for all living beings and protect them.
14.I shall not steal.
15.I shall not tell lies.
16.I shall not commit carnal sins.
17.I shall not take intoxicants like liquor, drugs etc.
18.I shall endeavor to follow the noble eightfold path and practice compassion and loving kindness in every day life.
19.I renounce Hinduism, which is harmful for humanity and impedes the advancement and development of humanity because it is based on inequality, and adopt Buddhism as my religion.
20.I firmly believe the Dhamma of the Buddha is the only true religion.
21.I believe that I am having a re-birth.
22.I solemnly declare and affirm that I shall hereafter lead my life according to the principles and teachings of the Buddha and his Dhamma.
Theyre teaching is based in Theravada Tradition.
Just for curiosity.
We live in a no different system trough.
Examples of the Indo-European Castes:
Indo-Aryan - Brahmin(priests/teachers/scientists/engineers), Kshatriyas/Rathaestar(warrior-kings), Vaishya(traders-craftmen), shudra(workers-labours)
Roman - Flamines, Milites, Quirites
Celtic - Druides, Equites, Plebes (according to Julius Caesar)
Anglo-Saxon - Gebedmen (prayer-men), Fyrdmen (army-men), Weorcmen (workmen) (according to Alfred the Great)
Slavic - Volkhvs, Voin, Krestyanin/Smerd
Nordic - Earl, Churl, Thrall (according to the Lay of Rig)
Greece (Attica) - Eupatridae, Geomori, Demiurgi
Greece (Sparta) - Homoioi, Perioeci, Helots
Then it was replaced into the class system
Modern Times - Ruling classes (Industrial and financial capitalists, landlords), Lower Class (the petit bourgeoisie, the peasantry, wage labourers). Some capitalists can classify this into six, but no matter what, it is the same.