Here’s an article giving a bit of history on the use of “they” and advocating for its use as a gender-neutral pronoun.
Letter-for-letter, no part of speech gets people more worked up than pronouns do. Linguistic history is dotted with eruptions of pronoun rage. Right now, the provocation is the gender-neutral pronouns that some nonbinary people have asked to be called by, so that they won’t have to be identified as “he” or “she.”