Singular they has been part of the English language since the 14th century.

Bodine (1975) – Androcentrism in Prescriptive Grammar

Excerpt of AI-generated findings:

Though published in 1975, Bodine’s landmark paper relies heavily on field data and structural analyses of English third-person singular sex-indefinite pronouns conducted during the late 1960s (specifically citing frameworks from Postal, 1969). Bodine categorizes this phenomenon into two main types:

• Sex-unknown: e.g., "Who dropped their ticket?"

• Mixed-sex, distributive: e.g., "Anyone can do it if they try hard enough."

Bodine notes that despite two centuries of intense prescriptive efforts by educators to enforce the generic "he", singular "they" remained the dominant colloquial and written choice for nonspecific referents throughout the 1960s.

Actual abstract from that article:

Abstract

This paper demonstrates that prior to the beginning of the prescriptive grammar movement in English, singular ‘they’ was both accepted and widespread. It is argued that the prescriptive grammarians' attack on singular ‘they’ was socially motivated, and the specific reasons for their attack are discussed. By analogy with socially motivated changes in second person pronouns in a variety of European languages, it is suggested that third person pronoun usage will be affected by the current feminist opposition to sex-indefinite ‘he’ – particularly since the well-established alternative, singular ‘they’, has remained widespread in spoken English throughout the two and a half centuries of its ‘official’ proscription. Finally, the implications of changes in third person singular, sex-indefinite pronouns for several issues of general interest within linguistics are explored. (Language change, sex roles and language, language attitudes, language planning, prescriptive grammar, pronouns.)