>You could argue that it's been in debated use, but not common use. Otherwise there wouldn't be a debate...
This is just like people claiming "ain't" is improper English, or that you shouldn't split infinitives.
Any kind of "debate" is irrelevant because language prescriptivism is nonsense. There's no authority on high to say whether something is correct or not. If many people use a construct, it is correct because they are communicating with it.
> Former Chief Editor of the OED Robert Burchfield, in The New Fowler’s Dictionary of Modern English Usage (1996), dismisses objections to singular they as unsupported by the historical record. Burchfield observes that the construction is ‘passing unnoticed’ by speakers of standard English as well as by copy editors, and he concludes that this trend is ‘irreversible’. People who want to be inclusive, or respectful of other people’s preferences, use singular they. And people who don’t want to be inclusive, or who don’t respect other people’s pronoun choices, use singular they as well. Even people who object to singular they as a grammatical error use it themselves when they’re not looking, a sure sign that anyone who objects to singular they is, if not a fool or an idiot, at least hopelessly out of date.
Just for the record, it's not me calling you a fool or an idiot or hopelessly out of date, it's the former Chief Editor of the Oxford English Dictionary. The only debate here is the politically charged one with fake grammar authority that you have made up in your head.
You fall into the trap of confusing your tiny enclave of English speaking cliques as being representative of the rest of the planet where English is spoken regularly.