I was taught this very aggressively in school, but it turns out that it’s just not true and there’s a long history of singular they.

Singular “they” actually predates singular “you”.

It doesn’t though. They has been used for singular without knowing the gender for a long time.

My friend forgot their umbrella at the office.

They nervously answered the phone.

They’re a lazy motherfucker.

I gave my friend a hug and wished them a speedy recovery.

Any of those sound unusual, or like they’re referring to more than one person?