[a-priori]

Yes, it is. It's called the "singular 'they'", but some native English speakers were taught that it's incorrect. The controversy over it seems to be an American thing, that at some point in the 19th century someone wrote in a style guide that it's improper grammar, and the meme stuck.

But it's perfectly correct: English writers have been using both the singular 'they' continuously for hundreds of years (14th century?). It also has equivalents in other west European languages (e.g. the German neuter gender and the French ils for a group of people of indeterminate gender), so it's not an English invention either.