This is not true. Using 'they' as a third-person singular is incredibly common, is becoming increasingly so, and has been in use for hundreds of years.
This is also the case in some Romance languages, and I've always been put off by presumptuous "he"s or "she"s in the ambiguous case when the writer could have just said "they".
Btw, worst option: "he/she"
Wtf xD