When I was in school too many decades ago, I was taught that the plural for deer was deer, not deers.
Deer Singular: "That deer is eating the tree."
Deer Plural: "Look at all the deer eating the tree." Not "look at all the deers eating the tree."
So why do we insist in drinking beers? It must be that majority Denny showed in his graph. The same rule is followed by the word beer or any other word with two ee.
Beer Singular: "I'd like to have a beer please."
Beer Plural: "Let's go out and have some beer." not "Let's go out and have some beers."
Thank you for indulging our needs for some grammar.
It's both. Beer can be used as both a mass noun, like furniture, so you'd say "let's go have some beer". It is also a singular noun, so you can have a beer or four beers. You wouldn't say to a bartender "I'd like four beer". Well, I guess you might and you'd get what you wanted, but she might look at you funny.