Just to set the record straight here (it is not likely to be crooked elsewhere), the declarative form of question has long been used in both speech and writing, and continues so. Use of the question mark (eroteme, note of interrogation) as terminal punctuation for declarative questions is standard, common, and, as far as it goes, 'correct'. Both of these examples,
How to punctuate a list of questions?
Where to put the periods when using a parenthetical sentence?
are converted into declarative questions by use of the mark; without the mark (or other suitable context), they would not be questions.
Prescriptive grammarians now and in the past endorse, and use, questions terminated with question marks in declarative (as opposed to interrogative) form. Goold Brown, for instance, who invented and adopted the term 'eroteme' (deriving it from 'erotesis') to refer to question marks, explicitly endorses its use for declarative questions in his 1851 (revised 1857) The Grammar of English Grammars. The work contains a number of examples of his own and others' use.
Obs. 4 — A question is sometimes put in the form of a mere declaration; its interrogative character depending solely on the eroteme, and the tone, or inflection of voice, adopted in the utterance: as, "I suppose, Sir, you are his apothecary?" — Swift: Burgh's Speaker, p. 85. "I hope, you have, upon no account, promoted sternutation by hellebore?" — Id. ib. "This priest has no pride in him?" — Singer's Shak., Henry VIII, ii, 2.
p. 769, bottom.
No less a contemporary prescriptive grammarian than Lynne Truss, in the seminal popular grammar Eats, Shoots & Leaves (2004), certainly uses declarative questions, repeatedly, even if she does not explicitly endorse their use (she might; I can't tell without actually reading her corpus):
Readers are obliged to get used to the idea from an early age that "Double or single?" is a question not applicable only to beds, tennis and cream. (p. 171, middle)
In the old days, we used to ask the following question a lot: "One word? Two words? Hyphenated?" (p.189, bottom)
Descriptive grammarians, of course, have no shortage of declarative questions in speech and writing to draw from, as is evident in, for example, a book chapter in Abduction, belief and context in dialogue: studies in computational pragmatics (2000, pp. 311-26) by R.J. Beun titled "Context and Form: Declarative or Interrogative, that is the Question":
You said Monday?
The italicized utterance by I is an example of a question in D-form [declarative form]. Here, the use of the declarative as a question is accentuated by the question mark.