Note: The below reference to Guru Hargobind Sahib is a contemporary eye witness account, so very valuable. The reference to earlier Gurus is clearly taken from hearsay as the author suggests the 6th Guru does not follow the precepts of the earlier Gurus.
Interestingly, the author does however state that Guru Nanak Dev ji banned pork, which is line with Guru Gobind Singh Jis rehits of domestic pig meat being banned.
Dabistaan-e-Mazahib, Mohsin Fani (1650s) translated by David Shea and Anthony Troyer (1843)
Having prohibited his disciples to drink wine and to eat pork, he himself abstained from eating flesh, and ordered not to hurt any living being.
After him, this precept was neglected by his followers; but Arjun mal, one of the substitutes of his faith, as soon as he found that it was wrong, renewed the prohibition to eat flesh, and said: “This has not been approved by Nânac.”
Afterwards, Hargovind, son of Arjunmal, ate flesh, and went to hunt, and his followers imitated his example.
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This is a truly panthic effort, we would be honoured to add your findings here. Please provide the exact reference and content in either Gurmukhi or English. Puraatan ithihaas, puraatan rehit or Gurbaani is foundational. But if you have any other useful and relevant references we will certainly consider it.