Singular "they". I don't care if it's been used for centuries. Obviously, it hasn't been used widely enough to be standard or accepted. Sure, you can make a case for singular "they" being used in casual conversation where clarity isn't that important, but not in formal writing. I don't understand the push for it when there are other words that can be used that aren't ambiguous.
On a related note, it pisses me off how linguistic prescriptivism is constantly shat on. I'm not against languages changing, but if they change in a way that makes communication less concise & more vague or ambiguous, then it isn't wrong to enforce the rules & fight back against the change, minorities be damned. Without the rules & conventions, we wouldn't be able to understand one another right now.