Since the concept of "alternate personal pronouns" was introduced.
"They" became the singular term when "he" or "her" was not appropriate because someone is neither. This was a goof in the evolution of the English language, and has since also become a singular for he or her. This allows it to be used when sexual identity is unknown, and allows writers to reduce their risk of being denounced as unwoke. We don't have an equivalent of LEcole Francaisein this country, but the AP Style Guide indirectly provides for a level of indoctrination.
I am not bothered by the LGBwhatevers having alternate pronouns, but I still find the use of "they"" as a singular the syntactic equivalent of fingernails on the chalkboard.