And Chaucer split infinitives, but I was always told it was "wrong" in gradeschool. That's the problem with pedantry: language is a fascinatingly complex and beautiful set of patterns. Boiling it down to rules is at best a handy style guide for formal writing, but at worst it gets weaponed as a way to discriminate against people who use lower prestige dialects.

That's true, and there is evidence of "they" being used as a singular as far back as over 700 years ago, but only within the last few decades has it been formally accepted by style guides, like the APA or the Chicago Manual.