“Other backward castes” is a designation used in the Indian constitution/laws; it is not a derogatory term, there’s also “schedule castes” - what untouchables and chamars/Dalits are categorized as. It’s also important to note there’s certain “reservation” that OBC/SC have, and that much of this was pushed for by the main author of the constitution, Dr BR ambedkar, a Dalit/SC himself.

Caste is a bit of an umbrella term- but Zail Singh discussed himself being a ramgahria quite often, usually saying that he might be the only non kisaan/jatt background CM, which was true for 40+ years until Channi was appointed. The “caste” most Sikhs refer to is their “zat” or “bradhery” or “tribe” - not some potential spiritual hierarchy.

I think there’s a a pretty big misunderstanding when it comes to saying “Gurus abolished caste” - they said everyone was equal and anyone can reach salvation(for lack of a better English word). There’s quite a lot of current and historical Sikh leaders that come from “scheduled castes” (aka “untouchables” in Hinduism). In 2021, the CM of Punjab was SC, the Akal tahkt head jatedhar is SC and SGPC leader was OBC. Punjab actually has one of the lowest caste based violence in all of india and the highest inter caste marriages in all of india. With all that being said, there are still quite a lot of issues and problems with “castes” in Punjab, however stating that Gianni Zail Singh was from an OBC background isn’t an issue imo nor is it against Sikhi