Was Buddha/did Buddha teach Vegetarianism?

This question is frequently asked, and the range of answers is always wide. It really comes down to a question of context. Do you really want to follow the Buddha’s heartfelt intent, or cling to an exception he granted to monks who begged for their food thousands of years ago, rather than buying it in a modern supermarket as is done today. Here are some relevant quotes so that you can decide what compassion means:

The reason for practicing dhyana and seeking to attain Samadhi is to escape from the suffering of life, but in seeking to escape from suffering ourselves, why should we inflict it upon others? Unless you can so control your minds that even the thought of brutal unkindness and killing is abhorrent, you will never be able to escape from the bondage of the world's life. No matter how keen you may be mentally, no matter how much you may be able to practice dhyana, no matter to how high a degree of Samadhi you may attain, unless you have wholly annihilated all tendency to unkindness toward others, you will ultimately fall into the realms of existence where the evil ghosts dwell. You of this great assembly ought to appreciate that those human beings who might become enlightened and attain Samadhi, because of eating meat, can only hope to attain the rank of a great Raksha and until the end of their enjoyment of it, must sink into the never ceasing round of deaths and rebirths. (Surangama Sutra: Importance of Keeping the Precepts)

The Buddha said, “Son of my lineage, my teaching is not like that of the naked ascetics. I, the Tathagata, established rules of discipline in relation to specific individuals. Consequently, with a certain purpose in mind, I did give permission to eat meat regarded as suitable for consumption after it has been subjected to threefold examination. In other contexts, I have proscribed ten kinds of meat. And yet again, with someone else in mind, I have declared that it is improper to consume meat of any kind, even of animals that have died of natural causes. But I have affirmed, O Kashyapa, that henceforth, all those who are close to me should abstain from meat. (Mahaparinirvana Sutra: Abstaining From Eating Meat and Fish, Even Died by Natural Causes)

Unfortunately, there are many who will try to persuade you to think that eating meat is ok, as long as you don’t kill the sentient being yourself. The Buddha foresaw this:

After my Parinirvana in the last kalpa these different kinds of ghosts will be encountered everywhere deceiving people and teaching them that they can eat meat and still attain enlightenment. But how can any faithful follower of the Lord Tathagata kill sentient life and eat the flesh? (Surangama Sutra, Ibid)

The root tantra of Kalachakra says: Wicked people, hard to train, kill harmless beasts as sacrifice to gods and for their ancestors, to gain protection, profit, and fulfil their aims. To buy the meat, to wish to eat it, is indeed an evil act.

The Buddha was quite clear that he taught compassion, not evasion of responsibility:

For innumerable reasons, Mahamati, the Bodhisattva, whose nature is compassion, is not to eat any animal flesh. I will explain the reasons: Mahamati, in the long course of transmigration, all sentient beings have been our brothers, sisters, mothers, fathers, sons, and daughters, and we have felt many different kinds and degrees of kinship with each and every one of them. These sentient beings have been beasts, domestic animals, birds, and humans in different lifetimes and have often been related to us in some way. This being the case, how can the Bodhisattva-Mahasattva who desires to respect all sentient beings as he or she would respect himself or herself and who is committed to devotedly practising the Dharma eat the flesh of any sentient being whose nature is the same as himself or herself? Even, Mahamati, the Rakashasa, when they listened to a discourse on the highest essence of the Dharma by the Tathagata, were inspired to protect Buddhism. Through this they had awakened to the feeling of compassion, became sensitive to the sorrows of sentient beings, and therefore chose to refrain from eating animal flesh. How much more should human beings who love the Dharma do the same! Thus, Mahamati, whenever and wherever there is evolution among sentient beings, let people cherish the thought of kinship with them, and holding the thorough intention of treating them as if they were our only child, and therefore refrain from eating their flesh. So much more should Bodhisattvas, who are committed to being compassionate towards all sentient beings, and whose inner nature is compassion itself, choose to refrain from eating animal flesh. For a Bodhisattva to keep good integrity with the Dharma, he or she should not make any exceptions to the eating of animal flesh. He or she is not to eat the flesh of dogs, donkeys, buffaloes, horses, bulls, humans, or any other sentient being whether or not such flesh in generally eaten by some humans in some culture or society. Nor should a Bodhisattva eat flesh sold by others for monetary profit.” (A Re-translation of the Eighth Chapter of the Lankavatara Sutra and Commentary, By William Bagley, 2006)

Pure and earnest bhikshus, if they are true and sincere, will never wear clothing made of silk, nor wear boots made of leather because it involves the taking of life. Neither will they indulge in eating milk or cheese because thereby they are depriving the young animals of that which rightly belongs to them. It is only such true and sincere bhikshus who have repaid their karmic debts of previous lives, who will attain true emancipation, and who will no more be bound to wander to this triple world. To wear anything, or partake of anything for self-comfort, deceiving one's self as to the suffering it causes others or other sentient life, is to set up an affinity with that lower life which will draw them toward it. So all bhikshus must be very careful to live in all sincerity, refraining from even the appearance of unkindness to other life. It is such true hearted bhikshus who will attain a true emancipation. Even in one's speech and especially in one's teaching, one must practice kindness for no teaching that is unkind can be the true teaching of Buddha. Unkindness is the murderer of the life of Wisdom. (Surangama Sutra, Ibid)

Whoever, having laid aside violence in respect of all beings, moving or still, does not kill or cause to kill, him I call a brahman. (Sutta-Nipata III.9:629)

These are just a representative example of quotes. There are many more. There are also reinterpretations of the Buddha’s teachings that stray from his heartfelt-intent. In those cases, ask yourself if he meant selective compassion, or compassion for all sentient beings. And follow your heart—don’t listen to others.

Maybe "Purification is not reached by food" helps in addition to given good answer by householder Ruben, good householder, but by ways of how one gains it and uses it, observing precepts of right resolve. And there is nothing wrong to chose (if still chosing) non-meat if it does not require harm by yourself, order others to harm, or approve harming for desired.

[OP:] However Mahayana Sutras state that the Buddha forbade eating meat for example in the lankavatara and Surangama sutras.

Then the burdens fall on whoever making the claim to provide the exact sutras' references to counter what was instructed in MN 55:

"In three cases I say that meat may not be eaten: it’s seen, heard, or suspected. These are three cases in which meat may not be eaten.

In three cases I say that meat may be eaten: it’s not seen, heard, or suspected. These are three cases in which meat may be eaten." ~~ MN 55 ~~

Was Buddha/did Buddha teach Vegetarianism?

The Buddha never taught vegetarianism. There's not any unwholesome Kamma involved in the eating of meat. Eating meat is just a biological/sensory process of chewing, swallowing, tasting, smelling and thinking (e.g. if one likes the flavour).

The Buddha taught that monks may not eat meat that has been killed/is suspected of having been killed only to serve the monk.

The 3-fold rule has already been mentioned, and it's intention appears to have been to minimise the slaughter of animals for food. In a modern context, eating meat seems ethically neutral, though buying meat might be questionable, particularly in the context of developing Right Intention and harmlessness.

It is popular opinion that eating meat is definitely killing karma because it indirectly causes the killing of animals. This was also Tissa's attack on Buddha Kassapa, a brahmin by birth.

However, from Buddha Kassapa's quote below, we see that eating meat by itself is not killing karma.

“Killing living beings,
hunting, cutting, binding,
theft, lying, fraud, deceptions,
useless recitations,
associating with the wives of others:
This is a raw stench,
not the eating of meat.
Snp 2.2

(and more similar statements in the whole text of the same sutta)

The reason for this, is that karma is intention in Buddhism.

A person who kills an animal or instructs another (like a butcher or hunter or chef) to kill an animal, clearly has the intention to cause the death of that animal. That's the karma of killing imbued with aversion (dosa) and delusion (moha).

However, a person who eats a meat dish (made from an animal that was long dead) only has the intention to eat meat and taste meat. So, that's the karma of eating and tasting meat imbued with greed (lobha). This is not the karma of killing.

Eating any food that was grown in farms or wearing cotton clothes harvested from cotton farms, are also fraught with the indirect killing caused by tillage, use of pesticides etc. Surely one eating rice or bread doesn't have the intention to kill animals either, by doing so.

"Intention, I tell you, is kamma. Intending, one does kamma by way of body, speech, & intellect.
AN 6.63

All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with an evil mind, 'dukkha' follows him just as the wheel follows the hoofprint of the ox that draws the cart.
Dhp 1