I fail to understand how is that different than let say...
"I read an article which I didn't understand".
Is it really that different than:
"I read an article about English grammar, which article I didn't understand" ?
"I read an article which I didn't understand".
Is it really that different than:
"I read an article about English grammar, which article I didn't understand" ?
Note, for example, that we definitely don't do it with "who":
"I saw a girl who was singing."
"I saw a girl, who girl was singing."
"Which" as an adjective is still relatively common. Consider, e.g., "Which one did you say you replaced?" It's the fact that it's being used as an adjective instead of a pronoun in this case that is unusual.
(Determiner is the part of speech to which the articles belong, but it also includes things like each, this, some, etc.)
"Determiners can be used as pronouns" isn't true either, though many of them can. See whether you think you can do it with every or a(n).
I'd just rephrase the whole sentence to avoid ambiguity if it were a problem because saying "which article" doesn't quite feel right in this context—a bit too formal and old-fashioned?—but this use of "which" seems like a reasonable way to disambiguate if it fits in context.
We drop the implicit noun just like we drop the imperative subject, i.e. we say "Close the door" now instead of "Close you the door".
Or even "Fly, you fools!"
Even in 1934, you didn’t think of it as eliding the noun when you used “which” as a pronoun; rather, you thought of using the noun again as an escape hatch for ugly situations like this. More to the point, in modern English, where we don’t have this option, we’d have to restructure the sentence to avoid this ambiguity, not simply drop the noun.
Since "which" is a pronoun, it can stand in for the actual noun, as it does in your first example. But sometimes the noun is restated along with the pronoun.
Here's another interesting example: "Which article didn't you understand?" In question form, the "ante"cedent of the pronoun would actually come long afterward, even from a different speaker, as the answer to the question. But to add clarity to the scope of the question, a noun summarizing the answer type ("article") accompanies the pronoun.
"I read something by Victor Davis Hanson today, which article I didn't understand".
But I like retro phrasings for their own sake.