As a fellow word nerd—you are correct, in certain circumstances “they” was always used, when the gender was unclear.

That said, any of us over a not-very-high age were thought to use “he/she” as a first person of unknown gender and told that usage of “they” was poor usage. It was, then.

Rules change and language evolves and here we are.

Some humans are reluctant to change with it.

I will say, as a writer sharing stories about kids using they/them pronouns, I’ve noticed the singular “they” can tangle my sentences. I’d guess I now use fewer pronouns and instead use “child/kid/kiddo” (or their name) for clarity.

Not complaining, just noting since we’re on the subject. Thanks for your comment!