The antiquity of 'they' as the indefinite singular is well established. "If you do ever figure out whose umbrella this is, do give it back to them, will you?" has always been correct when the identity of the individual (and therefore particulars such as gender) is unknown.

As a definite singular, one used to refer to a known person who relates to gender in a specific way, it is rather new. I have no beef whatsoever with this particular linguistic innovation, but let's not pretend that it isn't one.