Indeed, it has. But not universally; its use has primarily been limited to in certain contexts. To quote from the OED's definition of "they": "2. In anaphoric

Indeed, it has. But not universally; its use has primarily been limited to in certain contexts. To quote from the OED's definition of "they": "2. In anaphoric reference to a singular noun or pronoun of undetermined gender: he or she." OK, it certainly recognises a singular usage... "Especially in relation to a noun phrase involving one of the indefinite determiners or pronouns any, each, every, no, some, anybody, anyone, etc." Aha - so this usage is not (historically, at least) equally well established for all contexts. Indeed, the entry goes on to note that: "This use has sometimes been considered erroneous." So it's hardly surprising if some people feel a bit uncomfortable being told they should be embracing it more widely.