If you look at pg. 29 in this cocktail manual, The Reminder by Jake Didier, published c.1905 (no date shown) at pg. 29 a recipe for “beer sour” appears.
This is very interesting, it is: add four dashes of lemon juice in a glass and fill with lager beer.
A dash’s quantity will always be controversial, perhaps pedantically so. Take a lemon wedge, then do four, not over-energetic squeezes in a glass, you’ve got it. Pour in your lager.
Given the whole history of 19th century bottom-fermentation and of improving brewing technology to banish acidity, why introduce it in beer? I think the reason was, as lager replaced the often-tart ale, some people missed the tang of slightly-off ale.
So, put lemon juice in good beer. A similar trick was used earlier in Britain to make very new ale taste old, Seville orange, sulphuric acid, or something similar was added.
Didier’s book ran to a 5th edition, the one I linked is I believe the first and rather crudely printed and bound. The last edition I read, 1917’s, omits the “beer sour”.
Yet today sours are a successful category of craft brewing. Many people like the tart taste, and it has come back in this form. Belgian lambic and other sourish ales inspired the trend here. Albeit the taste had just barely survived in Belgium itself.
The lemon wedge with some wheat beer is a stand-by of course, although I think there it is not to balance a sweetness, but more to complement the wheaty taste.
Pre-Prohibition lager was quite malty, as I have documented earlier, and remained so in post-Repeal brewing until World War II. Some ale drinkers who couldn’t get ale easily probably figured out that lemon juice masked the sweetness and provided a tang they recalled in old ale, hence it becoming a bartender’s trick.
I’m not sure the panaché/shandy idea is similar as those are very sweet drinks but the lemony connection may mean something.
I don’t know about four dashes but acidity is a funny thing, you need a certain amount of it in any beer. I’ve added different acid agents and sometimes you wouldn’t know anything of the like was added, if you don’t add too much that is.
Anyway, purpose-made sours: what’s old is new again. However, it is safe to say the sour taste was even then, and is certainly today, the preference of a decided minority, indeed a minority within a minority where craft beer is concerned.