The Reinheitsgebot or the “German Beer Purity Law” is the world’s first food purity law dating to 1487 (although not codified until 1516). It allowed only malt, water & hops in beer production and regulated costs & quality of beer. Yeast was unknown at the time and not added to the list of allowable ingredients until the 1800s, thanks to Louis Pasteur. It ruled German beer production for domestic production until the EU overturned it in 1992, although many brewers continue to adhere to it and its principles (although not necessarily the original price controls).
That said, many large German brewers do use other ingredients, including adjunct, for their beers exported to other markets, including the United States.