The singular "they" is an established part of English that has been used by competent writers for centuries, almost as long as the singular "you", whose grammar it shares.
Okay, that is an interesting distinction. It might even be true. Although I don't think it's that new to also use "they" to refer to a specific person whose gender you don't know (perhaps because you haven't met them).
It is at least a reasonable claim that could be investigated further, as opposed to the claim that singular "they" is always an error, which any corpus will show is false.
> Although I don't think it's that new to also use "they" to refer to a specific person whose gender you don't know (perhaps because you haven't met them).
That's possible but I've never seen any historical example of this nor I have seen anyone do it (outside of a specific subculture) so I don't think it's true: after all it's usually easy to deduce the gender of someone you haven't met from their name and if you get it wrong nobody (outside of that same specific subculture) will get really angry because of that.
You probably use it daily without noticing.