I'd say that assuming 'he' might be a US thing - 42 years ago when I was in Australian university math | comp sci classes a third of the students were female as were staff.
Even then I routinely used 'they' when writing about people in general, authors I had not met, etc. as there was a good chance they weren't male.
It used to be taught that the singular "they" was ungrammatical. (Ironically, the singular usage predates the plural.) The rule faded in other parts of the Anglosphere a bit earlier than in the US.
While being no expert on the historical development of the english personal pronouns (I do read some old english and maintain some fluency in modern ditto, not my first language), the linked Wikipedia page clearly states the opposite: singular they came into use after the plural use.
This is a minor nitpick, as I suspect that third person personal pronouns where in a state of flux during the middle english period, replacing some inherited pronouns with pronouns borrowed from old norse. More so, language isn't defined by it's history but from how it is used presently!
I myself wouldn't use singular they, it goes against my “language intuition
” (probably formed by my native language which wouldn't allow that construction), others feel free!
Wikipedia reinforces my understanding .. it's been in common use for centuries and only relatively recently have a few dipshits declared it to be "wrong"
Singular they has been criticised since the mid-18th century by prescriptive commentators who consider it an error.
Who d'fuck gives a toss about prescriptive gammons tellin udders de write ways to use da Engrish, 'hey?
FWiW the Oxford English Dictionary is descriptive and not prescriptive.
