It is certainly not a recent phenomenon at all. There are numerous historical examples of vegetarianism and veganism. One of my areas is in dietary ethics in South Asia so I will just discuss some historical cases there. However, the Ancient Greeks also had examples of vegetarians, in particular Pythagoras.
In terms of ancient India we know for certain that there were a number of religious groups that practiced vegetarianism and sometimes even more extreme diets.
In the Pali canon the Buddha reports that he practiced various types of fasting which were considered amongst the orthodoxy to be instrumental in spiritual development. The view was that by depriving the body in various ways it was possible to remove spiritual impediments and this would hasten religious awakening (moksa). However, this kind of dietary deprivation was mostly spiritual and often did not have any obvious ethical implications as far as the animal is concerned.
The best example of this level of moral scrupulousness during the ancient period is the Jains who were a rival heterodox sect to the Buddhists. The founder of Jainism Mahavira required that his followers abstain from meat altogether. The view was that killing or even supporting the killing of animals would produce bad karma. Under the Jain view, this bad karma would attach to the soul and weigh one down preventing liberation which was understood as elevation to the highest plain of existence called siddhasila. The Jains took things even further, however, and Mahavira even asked his followers not to inform farmers or hunters of where animals are located for fear that they might use the information to kill the animal. So they take the killing of animals extremely seriously. As Basham notes, we know that some Jains deprived themselves of food to a point where they would starve to death. This was considered an act of martyrdom.
Moving on to the Buddhists one of the interesting things that we uncover is just how raw and personal the rift between the Jains and Buddhists were in respect to vegetarianism. Like the Jains, the Buddhists held the principle of non-violence to be a basic ethical constraint that also applied to animals. But unlike the Jains, the Buddhists did not make meat eating mandatory at this time.
We learn in the Pali canon that the Buddha was approached by a disciple named Devadatta who insisted that the Buddha make vegetarianism mandatory. The Buddha rejected this plea and reinforced a prior injunction that meat could be consumed if the person who ate the meat was sure the animal had not been killed for his benefit. This is a clear difference from the Jains who thought that this personal connection was irrelevant. Here, the Buddha seems to be saying that killing animals is immoral, but eating their flesh is acceptable provided it doesn't support the practice of killing animals.
Devadatta would later be ejected from the Buddhist order after an assassination attempt against the Buddha. One interesting thing we learn from Xuanzang, who was a 6th ce Chinese explorer who was sent to India to investigate the state of Buddhism, was that there appeared to be a splinter group of the early Buddhists living in an Eastern area of India who practiced the injunctions of Devadatta. This included a strict adherence to vegetarianism. So if his records are anything to go by, a renegade cell of Buddhists appear to have formed after Devadatta and the Buddha's death with the idea of continuing Devadatta's recommendations around diet.
Going back to the arguments between the Jains and Buddhists, we actually see in the Jaina sutras a number of passages where Buddhists are excoriated by Jains for their apparent indulgences which include meat consumption. We have to consider this accusation in context to the area when fasting and other deprivations were the norm. In any case, these Jain texts ridicule the Buddhists for what the Jains perceive to be overindulgence. Karen Lang has done a lot of good work on this question and I encourage anyone to have a look at some of her articles on these disputes. Katherine Ulrich has also written an excellent article on the same issue.
We should consider the fact that this rejection of meat eating also appears to be a feature which develops in the orthodox religions which we now come to see as Hinduism. However, much of these deprivations have less to do with the importance of meat abstention as a moral matter and more to do with it being a matter to do with spiritual advancement. Caste purity and pollution is also an important factor in context to the orthodox tradition and we discover in some of the Dharmasastra texts (which are of ancient origin) that meat is considered an impure contaminant for people of certain castes. C.F. Fuller (and many other scholars) have also pointed out that animal sacrifice has long been important in orthodox and Hindu ritual practices since Vedic times. So we see a certain degree of ambivalence around the moral relevance of meat eating and animal killing in the orthodox tradition. As an ethical practice it has always been associated with the heterodoxy.
Another note here is with King Asoka who seems to have ruled around 200BCE not long after the death of the Buddha, Mahavira and the other heterdox leaders. From inscriptions on the famous Asoka Pillars, which are still accessible in India, we quickly learn that Asoka expressly forbade the killing of animals and seems to have endorsed a restricted diet on the grounds of morality. There is much debate as to the original source of Asoka's disposition and the Buddhists are quick to claim that this was due to Asoka's supposed conversion to Buddhism. The reality is probably that Asoka was a pluralist who was courting a number of different religions. Since many religions of the time frowned upon animal killing it was quite a safe bet for him to ban it in these injunctions. It is also doubtful that these rules were enforced but it does give us an indication of the historical importance of animal non-killing and the knock on effects this had as far as diet was concerned.
The above is just a fragment of some of the historical issues that come to the fore in the case of India.
Edit - I neglected to mention that the Carvaka/Lokayata tradition who were something like scientific materialists may have had followers who were vegetarians. Because most of the texts in that tradition do not survive we don’t actually know why they were vegetarians and therefore can’t say if it was for ethical reasons or for some other justification such as health.