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.)
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.