Sikhi rejects caste in its doctrine as completely as any tradition anywhere. Langar, karah prasad, the common name, the amrit ceremony administered by five men of mixed background in 1699, are all mechanisms designed to end it.
Punjab has the highest proportion of Scheduled Caste population of any Indian state, around a third. Most Punjabi Dalits are Sikh or Ravidassia.
In a large number of Punjab villages there are separate gurdwaras, separate cremation grounds and separate localities by caste. Land ownership is overwhelmingly with the Jat Sikh community. The reservation of a third of village common land for Dalit cultivation has to be fought for at auction, village by village.
The Ravidassia declaration of a separate religion in 2010 came out of this and not out of theology.
The Sikh Light Infantry exists as a separate regiment from the Sikh Regiment because of caste recruitment.
This is the single largest gap between what the tradition says and what the society does, and no account of Punjab that leaves it out is worth reading.
The teaching rejects caste explicitly and at length, and the practice has kept it. Separate gurdwaras, separate cremation grounds and endogamous marriage are ordinary facts of Punjabi village life.
The Ravidassia and Ad Dharm movements, and the Valmiki institutions, grew because the promise was not kept and the communities built their own.
The gap between the text and the practice is the oldest internal argument in the tradition and the one least discussed in public by the institutions.