What damage is caused to kefir when it comes into contact with metal?
The damage is not so much as to the kefir but to the consumer of the kefir if it was made in a reactive metal. Kefir, being acidic, should not be in aluminum, brass, iron or copper as they react to acid. Stainless steel is preferred because it is inert to the milk kefir is made from. However, when first made stainless was unknown.

Unless you have a real goat skin, about the best thing to make kefir in is glass. The "grains" are really a mix of yeasts and bacteria, and those can live in as much as a very light scratch in plastic. For this reason you should use glass to ferment it, and you can use a stainless spoon to stir.