It's just archaic English grammar, like using thou or sayest.
Like English spelling, it's correct (for an earlier version of the language).
That is a complementizer that marks a Tensed Subordinate Clause in English. It used to be able to appear in any kind of tensed subordinate clause -- noun clauses, adjective clauses, or adverb clauses -- as long as they have a subject, and a verb in the present or past tense.
This is still true for noun clauses (object and subject complement clauses), and for adjective clauses (restrictive relative clauses and NP complements), but not for adverbial clauses like unless I go with you.
but not in adverb clauses, at least not in Modern English.
- *When that I get back from vacation, we'll paint the garage.
But in Middle English it was OK.
(That is normally optional, though under certain conditions it is required. In the above, optional that is parenthesized. Probably it was optional for Chaucer, who was a poet and needed a handy store of optional syllables. Just like Cory Doctorow, at least in that respect.)
The that in Chaucer's Prologue is not a demonstrative that -- Which April? That April, or the other one? -- rather, it's filling the slot where one used to mark a tensed time adverb clause, right after the wh-word that indicates time.
A more modern (though regional) example is provided by Andy Griffith's comedy monologue, What It Was, Was Football, delivered in the comedian's native North Carolina dialect, which includes a number of archaizing features, including using that in tensed adverbial clauses.
About 12 seconds into the recording in the link above, Griffith says
- Different ones of us thought that we ought to get us a mouthf'l to eat
before that we set up the tent.
Since unless also introduces tensed adverbial clauses (a counterfactual conditional clause in this case), the rule would apply to it, too. In this dialect. But outside Modern English dialectal speech, it's pretty rare, so doing it in print is an affectation. Which is pretty typical of Cory Doctorow.
The word ox comes from the Old English oxa. In Old English, as in Indo-European languages in general (historically and even today), the number of a noun (singular or plural) and its function in a sentence—whether it was the subject, direct object, indirect object, or had some other relation to a verb or another noun—was largely (not solely) governed by sets of endings tacked onto it, or changes made to the vowels in it. These sets of endings or changes were called declensions, and each type of relationship associated with an ending is called a case.
There were a number of declensions in Old English; the two most prominent were the weak declension, containing the weak nouns, and the strong declension, containing the strong nouns. Old English oxa was a weak noun. The forms that we have of its descendant today are derived from the nominative case endings; these are the forms that would indicate that a noun is the subject of a sentence, or the forms that would be used when writing a list of nouns.
Since oxa was a weak noun, its plural form (the nominative plural form) was oxan. Over the course of centuries, the a "weakened" to an e, giving us oxen.
Fox, on the other hand, comes from the Old English fox, which was a strong noun; its Old English plural was foxas, whence we get foxes.
The source I used to confirm the declension of fox has an entry for the Old English box; however, it has no declension information. Using this translator, however, it appears that the nominative plural was boxas, giving us boxes.
Although Modern English has largely dropped the declensional suffixes we got from Old English, we occasionally see them peeking through, as we do here.
Best Answer
I take the real question here to be: "Why do some people pedantically cling to dying forms?" That's a good question. I think the answer is relatively straightforward.
People who want to present an air of education and in general lay claim to upper class privilege are the ones who tend to do this. It's largely because it is an index of education and a high degree of literacy (either that or role playing games, which is somewhat different, but still primarily an upper middle class past time...).
There is a lot more to it than that, of course. There is an intricate set of language ideologies which give rise to this kind of behavior. But the short answer is that in using such forms people attempt to identify with culturally and economically powerful (hence linguistically conservative) groups .
Everyone does this to some degree, of course.