Why is 19th century "traditional" while after WWII is "modern"? (And by 19th century, you could be talking 1894, for example, for Spaten Helles.)
As for undermodified malts ... They. Are. Just. Worse. They're a curiosity for brewers who are interested in the history of brewing, but historical re-creation is really the only reason to use them. Decoction? Single, maybe (though I'm not convinced) -- there's at least an argument to be made that Maillard reactions change the wort. More than that, again, is for history buffs but not brewers.
But yes, if it's a lager with a clean profile, relatively little roast (and smooth), some bitterness, and sessionable, call it a Schwarzbier, regardless of the ingredient list. If your friends (or customers) know what to expect when they hear "Schwarzbier," you only confuse things by calling it a German-American Dark Lager, or whatever.
As for undermodified malts ... They. Are. Just. Worse. They're a curiosity for brewers who are interested in the history of brewing, but historical re-creation is really the only reason to use them. Decoction? Single, maybe (though I'm not convinced) -- there's at least an argument to be made that Maillard reactions change the wort. More than that, again, is for history buffs but not brewers.
But yes, if it's a lager with a clean profile, relatively little roast (and smooth), some bitterness, and sessionable, call it a Schwarzbier, regardless of the ingredient list. If your friends (or customers) know what to expect when they hear "Schwarzbier," you only confuse things by calling it a German-American Dark Lager, or whatever.