Under the German, actually originally Bavarian,
Reinheitsgebot the grain choice is limited to barley, and only barley. Humans have been making fermented grain beverages for millennia.
Using corn lowers the cost of the beer, but if one uses the whole grain, it is not without flavor impacts. Rice adds significantly to the cost of beer, but has minimal impact on flavor. For brewers trying to have a cleaner label, rice is often preferred "purified corn starch", but not every brewer or consumer is worried about the list, but if you are curios, here is a nineteen page listing of the various things that turn up in just Coors' various beers.
With regard to IPAs, cost is less of a concern since yeast genetically modified to make hops flavors were allowed for use in beer, making IPAs potentially similar in cost to normal beers, if large amounts of real hops aren't added.
Hoppy beer is all the rage among craft brewers and beer lovers, and now UC Berkeley biologists have come up with a way to create these unique flavors and aromas without using hops. UC Berkeley scientists have found a way to avoid expensive, variable ...
news.berkeley.edu
Oregon State University brewing researchers and a team of bioengineers have shown that a genetically modified yeast strain can alter the fermentation process to create beers with significantly more pronounced hop aromas. “These findings could be extremely useful in creating new beer flavors and...
www.craftbrewingbusiness.com
A growing number of US brewers are now using GM yeast to make their beers.
www.bbc.com
The hops engineered yeast has been used for some time, but I don't have an exact date.
Personalky, I'm a fan of beers with just four ingredients; yeast, barley (malt), hops and water. That's not to say that I haven't brewed things like melomel, but I don't think of them as beer.
All the best,
Peter