> TL;DR: It's not a recent usage; it was OK in 1375.
Forgive me for not giving a detailed play by play of 15 years of
experience specifically as a writer and editor.
Over the last handful of decades, singular "they" has been explicitly
taught as inappropriate. My own college writing classes (only 10 years
ago now) included this specific piece of advice.
My college writing class in 1985 or so DID NOT eschew singular they. I've been a professional writer for about 30 years now. I am happy to stipulate that your class in 2010 at some particular college included an instructor saying "don't use singular they" ... but that was not uniform across universities in 2010, probably not even across the entire faculty at your particular school.
I'm 55 yo, and I remember 50 years ago hearing the nonsense claim that "singular they" is "bad feminists trying to corrupt the English language." I probably didn't know the 14th century origin of the use until a decade or two later than that, but this identical discussion was already extremely old by the time you were born.
In terms of modern English vernacular, singular "they" has been
continuously and rigorously treated as inappropriate.
This is absolutely and categorically false. There have been SOME PEOPLE who didn't like the singular they, starting about 1820. The idea never occurred to anyone during the first 450 years of its use. It has also never been uniform opinion at any point in the last 200 years. But as I say, some text books, and quite possibly your particular instructor at some particular school, was of that opinion.
Strunk and White, in current editions, does not hold that position.
Those who prefer singular "they", myself included, point to references
very much like yours as evidence that it has a long history of usage.
But until only the last few years, the popular style guides explicitly
forbade it.
Again... SOME guides. Except the ones that didn't do this.`
The dead increasingly dominate and strangle both the living and the
not-yet born. Vampiric capital and undead corporate persons abuse
the lives and control the thoughts of homo faber. Ideas, once born,
become abortifacients against new conceptions.