That hasn't been universally accepted yet. Grammar Girl has a good blog on it.
Sorry but I can't see this subject in her "Quick and Dirty Tips for Better Writing"
It may not be universally accepted, but it at least has some history to it with uses of a singular they going back at least to the 15th century
Whilst I appreciate the support (if that it be) the examples (note the past tense in the introduction):
Alongside they, however, it was also acceptable to use the pronoun he as a (purportedly) gender-neutral pronoun, as in the following:
"Suppose the life and fortune of every one of us would depend on his winning or losing a game of chess."— Thomas Huxley, A Liberal Education (1868); quoted by Baskervill.
"If any one did not know it, it was his own fault."— George Washington Cable, Old Creole Days (1879); quoted by Baskervill.
"No one shall be arbitrarily deprived of his nationality nor denied the right to change his nationality."— Article 15, Universal Declaration of Human Rights (1948).
in each case start by making them clearly all-inclusive.
References to he/she were made in the 1800's:
nineteenth-century grammarians insisted on a singular pronoun on the grounds of number agreement, while rejecting "he or she" as clumsy.
(from the above referenced Wiki page)
This followed the mid 1700's item
A recommendation to use the generic he, rather than they, in formal English can be found as early as the mid-18th century, in Ann Fisher's A New Grammar,
so there is some agreement even if it was from a woman who IN THOSE DAYS would not have been regarded as the fount of all knowledge. As the first published English language grammarian she did not cite others nor is she apparently widely cited.
I ignore the next paragraph referring to Ostade because she, a Netherlands lady, is merely reciting from 18th century female grammarans, presumably Ann Fisher
In conclusion, whilst there are arguments for using the masculine when referring to a person whose gender is not disclosed is the writer not thereby offending ladies in the age of claimed equality? Worse, the idea of "he or she" (or a modern equivalent he/she) has itself been condemned.
Edited to make the quotes clearer