Several, though not "holy" in itself, but befitting a holy person (saint-like behavior).
In this particular instances he means in the practice of Christianity, which in the Catholic and Orthodox version (or the unified version before the schism) had strict lent period, with rules about abstaining from meat (and, for those following the stricter rules, dairy products two in most days (and in the traditional versions, a bread+water diet only on some important holy days). Regular christians followed (and follow) those at their own preference/faith/tolerance level, the clergy, monks, and saints, follow them strictly. And there are several stories of saints who followed them year round.
But of course, veganism is also associated with holy behavior in Hinduism, Buddhism, and such.
To add to the above, early medieval fasting rules in Christianity were brutal, arguably much tougher than comparable rules from other major religions at the same time. Then we got decadent.
"Noon" is at lunchtime now because the ninth hour (Nones, Noon, 3pm) prayers, after which one could eat, got moved inexorably earlier to cater to the whims of impious monks.
And incidentally, because it's interesting, fasting and veganism without corresponding prayer was/is seen as Satanic, "The fast of the devils", because "fallen angels neither eat nor pray".
A lot. for example: jainism, zen buddhism (shōjin ryōri), rastafarianism. There are also a lot of religious practices where the religious doctrine doesn't _require_ adherence to a vegan diet, but not eating animal products is considered more holy, and specifically because of the abnegation aspect the post is talking about. Catholics, for instance, have a long and complicated tradition of avoiding animal products for religious reasons: https://en.wikipedia.org/wiki/Fasting_and_abstinence_in_the_...
