People who don’t think “they” can be used to describe one person:
⚫️ James Woods

People who have used “they” to describe one person:
💙 Jane Austen
💖 William Shakespeare
💮 Geoffrey Chaucerhttps://t.co/ZjKSKRjv0Z

— PinkNews (@PinkNews) February 16, 2019

TL;DR: James Woods argues against using “they/them” pronouns for non-binary folks on the basis that “they/then” has always referred to the plural. The official Twitter account of dictionary.com responds that “they/them” for a single person has been going since 1300, and people who have used this include Shakespeare.