Beer is a defined beverage. Stated differently, when a product is sold as an unflavored beer, it can contain only barley, malt, hops, and, in some instances, rice. The proportions of those ingredients, the geographic origin of them, the specific strains of various plantings, the water, and the brewing process are the variable factors that cause one beer to taste different from another.
But, the long and the short of it is that “beer” means a fermented beverage brewed from the above-noted grains. Anything else is not beer.
This is similar, in some respects, to the manner in which ice cream and chocolate were viewed in the U.S. in the 1950s. At that time, ice cream meant cream, sugar, and perhaps some natural flavor; chocolate was cocoa, milk, and sugar. A survey of people in their 70s and 80s will likely disclose that the accepted practice among frum yidden was to enjoy ice cream and chocolate without a hechsher (for those who eat cholov stam). But at some point in time, various additives were thrown into the mixing bowl, and the practice of eating any ice cream or chocolate came to a screeching halt. Fortunately, beer has not been subjected to such innovations, except to the extent that lime and lemon and other oddities are being added to the classic shalosh seudos beverage of choice. It is, to a purist, somewhat sinful, and conjures the same nose-wrinkling difficulties associated with flavored coffee.
Actually, coffee and tea may offer a fair comparison to beer – it’s coffee, it’s tea, it’s beer.
At one time, incidentally, fish by-products were used to skim some of the scurm from the fermented beer (during the brewing process). But that process was acceptable because the fish by-products (dried dead fish parts) was scattered atop the mixture and then skimmed off, along with the unwanted foam from the beer. So, nothing from the dead fish remained in the beer; the point was to actually pull all of that stuff out, along with the top layer of brewing beer.