https://en.m.wikipedia.org/wiki/Singular_they#Usage
> They with a singular antecedent goes back to the Middle English of the 14th century... and has remained in common use for centuries...
What I said is correct. Prior to about 4-5 years ago you did not find people using they in the singular, hence "in common use".
It is grammatically plural, but it has been accepted for semantically singular use since long before the Victorian effort to impose Latin-inspired rules on English usage which failed to eradicate it despite intense effort. (Victorian prescriptivism did have some good effects — regularized spelling FTW — but trying to eradicate clear and useful usages like singular “they” was one of its less-well-considered, but fortunately also less-successful, efforts.)
Nothing in what you wrote contradicts what I stated. The man on the street thinks they is plural, and nearly all writing treats it that way. Historical examples excepted.