It was incorrect English grammar when I learned grammar. It is still incorrect English grammar in some places. For reasons that cannot be discussed in these forums, American English is on the front lines of a ideological battle. Things are getting very ugly where I live and I am sincerely fearful my country will soon descend into civil war.
Don't make the mistake of seeing language as just whatever is taught in any given school or by any given teacher. English Grammar is not dogma, nor hard science. And "formal grammatical" English is not all there is to English, just a subset of it. Accepted colloquial English is not "wrong", either.
There's always been a hard nosed group of English (or any language) grammar police who, sadly, sees language as something that can be controlled with an iron fist, and it's perhaps not a coincidence those are also common among people who would like to meddle into the personal affairs and thoughts of others.
Actually, I doubt VERY much that everyone who shows grievance with this usage because it's allegedly "grammatically incorrect" always writes or speaks English in a gramatically correct manner. It's mostly just an excuse to justify prejudice against a more inclusive usage of the language.
You don't see anyone complaining today about "you" being used in the singular instead of "thou", but if there was social media in the 17th Century I bet we'd have seen the same kind of ruckus about it being "gramatically incorrect". The very same arguments against the usage of "they" in the singular exist for "you". The difference is the singular "you" doesn't bring to attention another totally different issue, which is recognizing and adressing our society's flawed construction of gender roles.
Someone saying that embracing a more inclusive usage of language might bring about a "civil war" is, quite frankly, mindboggling. Language has no feelings, English will be fine despite your worries.
Cheers,
NH