Indeed. I was taught very directly that the singular pronouns were he, she, and it. The plurals were we, you, they. So grammatically if you were referring to a singular person of unknown sex, then you should use "it" .

Obviously using the pronoun "it" at some point became offensive, so is highly not recommended. But, (probably after having drilled into my head repeatedly that "they" is a plural) it seems very incorrect to my ears when "they" is used to describe a singular person. It also unfortunately comes with ambiguity sometimes. I've had misunderstandings where I used they as a singular pronoun to describe someone of unknown gender, and the person I was talking to took it as a reference to a plural, which at best creates confusion, at worst misleads.

Language is an incredibly hard problem, and it certainly doesn't help that as youth, we are drilled with supposedly objective truth regarding language, when in reality it is far less defined and more nebulous than than the teachers would have us believe. The generational gaps can already be tricky to navigate. Having different ideas of objective truth, especially regarding language, certainly does not help.