Singular Pronouns You and They, previously plural pronouns

‘You’ was once just a plural pronoun with its singular version being ‘thou’. Nowadays, ‘you’ can be (and mostly is) used as both a plural and a singular pronoun. No one denies this. Hence there is nothing grammatically wrong with pronouns shifting. (Any linguist knows pronouns are the shiftiest of word types.)

‘They’ is officially a plural pronoun but it has been used as a singular pronoun at least since Shakespeare’s time. If you deny that ‘they’ can be singular pronoun then you forfeit the right to use ‘you’ as a singular pronoun, too. Do you use ‘thou’? No you do not.

There is no reason why ‘you’ is allowed to be singular but ‘they’ is not. If your reasoning leads to two different conclusions then it is senseless and illogical.

So yes, from a purely grammatical point of view, ‘they’ is a singular pronoun.