German Groups of Things

No yeast, no beer... The current german "purity law" allows yeast (obviously). There are no "original" rules, just many different rules in different times and different regions of today's Germany. The older ones don't mention yeast, as until about the 16th century people didn't know its essential role for brewing beer (some of the rules even don't mention water!). People then only recognized that bakeries were most successful in brewing. But they thought it to be a miracle by the gods... Today everyone knows yeast to be indispensable and to notably define the taste of beer. I definitely would add yeast to the list, because: no yeast, no beer ;-)

I think the question refers to the purity law of 1516, which is commonly considered the original or definitive one. Under that law, only hops, barley malt and water were allowed. Yeast was not mentioned because people back then didn't know about yeast, they used spontaneous fermentation. But of course, yeast is allowed nowadays.

Personally, I would not take that if I were you.

If anyone is wondering (like I was), Schneider means tailor.