There have always been vegetarians, and there have also always been omnivores (though most earlier cultures ate far less meat than we do today). Neither way is more “correct” or “ancient” or, indeed, spiritual, although many are likely to disagree with this statement.
Those in the West who feel that vegetarianism is necessary for magical and spiritual work are correct—for themselves. If they’ve made this commitment, it’s best that they keep it. No one can make a commitment for another, however, and no one way is satisfying for everyone.
Life feeds on life. Our bodies cannot survive unless something else gives up its existence to sustain us—whether it’s plankton, soybeans, or a chicken. This may seem cruel, but it’s not. It’s the reality of physical existence.
What you decide to eat or not to eat isn’t as important as why you make this decision. If you’re vegetarian because you feel that it’s the only way in which you can achieve any form of spiritual enlightenment, fine. If you’re vegetarian because you’ve decided that you can’t practice magic if you eat meat, again, fine. But others can make alternate decisions. They can decide to be omnivores, achieve spiritual enlightenment, and still successfully practice magic. Neither position, once again, is correct for all.