The Oxford English Dictionary traces singular they back to 1375
Use of the singular “they” is endorsed as part of APA Style
regardless of what detractors say, nearly everyone uses the singular they in casual conversation and often in formal writing
does not prohibit the use of singular they as a substitute for the generic he in formal writing
[singular they] has emerged as a tool for making language more inclusive . . . and the MLA encourages writers to accept its use to avoid making or enabling assumptions about gender.
Everyone Uses Singular 'They,' Whether They Realize It Or Not