The fact that this 1930-era additional meaning is being described as "new" makes me think that there might also be another meaning of "new" that I wasn't previously aware of!
My current favourite instance of singular 'they' is in the KJV translation of the Book of Job – specifically, the end of chapter 15. The authors used generic 'he' throughout, until they got to a bit they'd have to translate as "his womb". Clearly this was a wee bit radical of a concept for the authors, because they chose instead to write "their belly prepareth deceit".
Also, since it's a religious text, this is a slam dunk counterexample for (e.g.) prescriptivist Mormons. Anyone can handwave Shakespeare, but the Inspired Word of God? They have to admit that singular 'they' is grammatical.
Some churches have the doctrinal position that particular translations were divinely inspired. Indeed, there are people out there who will tell you that the King James Version is the _only_ 100% true and accurate Bible in any language, because God influenced the translators to correct errors in their source material.
(Most other churches think this is extremely silly.)
The antiquity of 'they' as the indefinite singular is well established. "If you do ever figure out whose umbrella this is, do give it back to them, will you?" has always been correct when the identity of the individual (and therefore particulars such as gender) is unknown.
As a definite singular, one used to refer to a known person who relates to gender in a specific way, it is rather new. I have no beef whatsoever with this particular linguistic innovation, but let's not pretend that it isn't one.
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